Before considering in detail the teaching of the Article it is necessary to enquire into the nature of sin as moral evil. For this purpose we must seek to know what is the essential moral characteristic of man. What is it that constitutes him a moral agent?
It is, first, his conscious relation to law, emphasis being placed on the consciousness, since, of course, all beings are subject to law. But man is sensible that in not acting up to it he is imperfect and guilty. Law applies to inanimate and also to animate natures, and it is in connection with the latter that man’s moral attitude to God and his fellows is seen. But in proportion as man’s spirit, soul and body form one being, the law has to deal both with inanimate nature and the ordinary animal nature. In regard to man law is concerned with his relation to the Law-giver, for law is the revelation of man being in contact with another and higher Will. This is the simplest form of the idea of God in the heart, and on this basis alone arises the duty of natural religion. Then, too, law concerns man’s relations to his fellows and to the world around him, and it follows from this that the perfection of our own nature is blessedness, since there is such a thing as an ideal for our life as that which is dependent on our true relation to God and man. Law is either naturally discerned or supernaturally revealed, and the Apostle Paul insists upon both of these (Rom. 1:18; 2:15).
But it is necessary to take another step. Men are not only conscious of law, but of responsibility to obey it, and this is the evidence of freedom of will which rests on the double basis of our own consciousness and the collective consciousness of man as seen in language (“you ought”), in institutions (laws), and in all religions.
Yet again, man not only has this consciousness, but also a conscience, a further and higher faculty, perpetually bearing witness to his obligation to use freedom in obedience to the law of his nature, whether declared by nature or revelation. Conscience has been called the “Categorical Imperative” (Kant). But this Imperative must be distinguished in two ways: sometimes it applies to the general principle of doing right; at others to the specific dictates or application of general principles of right and wrong. In determining this the co-ordinate faculties come into play, particularly the reason, and so this sense of duty is capable of indefinite enlargement.
Now these three facts are inherent in man’s nature everywhere. They are antecedent to revelation and are recognised without its aid. They may be regarded as the basis of natural religion and ethics, and are the elements of man’s normal state, as it ought to be.
But when we pass to man’s condition, as it is, we come to the momentous question of moral evil, though here again we are not dependent on revelation for the fact of its existence. Nothing is so prevalent as this fact in all religions, for there is a universal consciousness, exemplified in history, confessed in literature, and experienced in life, that man is out of harmony with the law of his nature. The certainty and consciousness of this in man is a characteristic of him in relation to other animals, for of none else can it be said that they are out of harmony with the law of their nature.
It is striking that testimony is available to show that man acquiesces in the state he finds himself, and thus, original evil is acknowledged by all. When we say evil we do not mean in the full sense sin, for these are two aspects of evil to be distinguished, even though they cannot be separated. Evil may be either an unconscious or a conscious violation of law. Beings born corrupt, inheriting a certain taint and bias of will are partakers of evil which did not originate with the will. But another form originates with the act of the will itself, and then we have sin in the proper sense. Children are born with an evil nature in a state of what is called depravity, and when reason dawns they know something of right and wrong, though they only have a partial responsibility, but in course of time they become fully responsible for the sin of their own will. Adam was placed under law, and disobedience was sin. When a further law was given under Moses, disobedience again became sin and involved personal guilt, but with those who were not thus brought into contact with the law sin was not imputed or counted as guilt, though its consequences remained. So that evil has a double aspect, physical and personal. Physically, wrong-doing entails inevitable consequences; but, personally, it is not imputed as guilt so long as there is no clear revelation of law. But directly the law is recognised it is imputed. Human nature, as Butler points out, in its essential idea is a balanced constitution, and he shows that through sin every part is impaired. It is this that constitutes what the Article calls Original Sin.
The English word “sin” seems to be allied to the Latin sons, meaning “guilty,” “sinful,” and apparently the origin of the Latin term is “real,” from the present participle of εἰμί, “I am.” “Language regards the guilty man as the man who it was” (Curtius).  It is also worth while to distinguish between vice, crime, and sin. Vice is wrong-doing against our own nature; crime is wrong-doing against our fellows; sin is wrong-doing against God.
At this point it is necessary to observe the more important words found in Scripture for sin. The most frequent is ἁμαρτία, “error,” “missing a mark.” Others are παράβασις, “transgression,” “crossing a boundary,” and παράπτωμα, “fall,” “to drop by the wayside out of a proper path.”
It is essential to distinguish between “sin” and “sins,” between the principle and the practice, the root and the fruit. This distinction is seen in Rom. 1:19 to 5:11 (sins) and Rom. 5:12 to 8:39 (sin), and also in 1 John 1:8, 10; John 1:29 with 1 John 3:5. Original sin has to do with the former of these, the evil principle, the root within our nature. 
The phrase “original sin” is not found in Scripture, and is thought to have been due to St. Augustine in the fifth century. It is not the most accurate phrase to employ, especially because the Article speaks also of “original righteousness,” and there cannot be two things “original.” Perhaps a better term would be “inborn sinfulness,” referring to that principle of evil which has infected human nature by reason of the original connection of the race with Adam in contrast to actual sins which men themselves have committed. It is an endeavour to go behind the sinful acts and to explain the fact that all men possess that wrong element which the Bible calls sin.
The Article makes no reference to original guilt, and this is sometimes said to be due to the fact that guilt is personal, while sin is in the race. But it should not be overlooked that the phrase “original guilt” occurs in Article 2, and something like this seems to be the truth of Holy Scripture. Indeed, a modern writer holds that the phrase “original guilt” balances the language of the Ninth Article, and represents much more nearly the dominant idea of the New Testament, and that guilt rather than sin “emphasises the fact that Christ’s relation to sin in its social aspect is precisely the same as in its individual manifestations.”  It is probably more correct to say that both guilt and sin are true, the former being imputed and the other imparted. Certainly the force of Rom. 5:12 (Greek) seems to indicate this. And if it should be said the imputation of guilt is unreal and impossible, it may be shown to be met by the imputation of righteousness, which on any ground is part of our Lord’s redemptive work on our behalf. There is, therefore, no injustice, or even unreality in speaking of original guilt, since it is met and more than met by the provision of Divine righteousness in Christ. If one is true so is the other. Adam’s posterity stands just where he stood after the Fall. The “probation” of the race was at an end when its first parent fell. And now Christ, the last Adam, meets and more than meets the sin and guilt of the first Adam (Rom. 5:12-19; 1 Cor. 15:22).
The Article first defines original sin negatively, as not consisting in copying Adam’s example. “Original sin standeth not in the following of Adam.”
Then it is defined positively as the defect and corruption of the nature. “But it is the fault and corruption of the nature” (vitium et depravatio). This inborn sinfulness is not only deviation, but deliberation; not mere absence of ethical vitality, but the positive presence of disease. As such, it is therefore unnatural in the sense that it was originally no part of human nature. 
Thus sin, while primarily a matter of the will, is very much more. No doubt in the strict sense of the word “sin” means “voluntary surrender to evil,” but the fact goes very much deeper. It is “the propensity to evil in individuals which seems to be inexplicable from anything falling within the individual’s own life.”  It is this that the Article emphasises as something far deeper than either act or volition. It is the presence of a moral disturbance in our nature, and concerns the dispositions and tendencies before the will begins to act. The tendency is there antecedent to our consciousness, and can rightly be called sinful.
“By Original Sin then seems to be meant the solicitations of the lower nature conceived of proleptically as sin, because, as present in the nature of a rational or moral being, they constitute the potentiality of the sin, which consists in such a being’s yielding to them, despite the consciousness that to do so is wrong.” 
The Article refers to the Pelagians, and it is essential to know a little of what Pelagianism means. During the first four centuries theological controversies were concerned with the Nature of God and the Person of Christ, and it was only after these questions were practically settled that Christian thought became directed to the personal aspects of truth. All along, however, the results of the Fall and the necessity of grace had been emphasised, but it was only in the fifth century that the subject of sin came into prominence in connection with the heresy of Pelagius. In order to emphasise free-will he denied the ruin of the race and the necessity of grace. This was not only something novel, it was really opposed to vital Christianity, and the struggle was soon seen to be one for the very life of the Gospel. The fundamental principle of Pelagianism is the assumption of human ability to do all that righteousness requires, and thus to provide not only its own salvation, but even its own moral and spiritual perfection.  From this general position the following results of the teaching of Pelagius were soon seen:
(1) Adam was created mortal and would have died if he had not sinned. Contrast “lest ye die” (Gen. 2:17; 3:3).
(2) The sin of Adam hurt only himself.
(3) Infants are, therefore, just as Adam was before his fall.
(4) Man is able to keep God’s commandments if he will.
(5) And so, all men may be sinless if they choose, and many saints even before Christ actually lived free from sin.
Thus, Pelagius denied the whole doctrine of inborn sinfulness, and with it the belief that man needed supernatural help for the purpose of obeying the Divine commands. The tendency of Pelagianism was twofold: (a) to make sin a matter of isolated acts, and therefore entirely separated from what preceded and followed. But it is impossible to ignore the continuity of life and to reduce man’s nature to a number of disconnected voluntary acts. It is obvious that if sin is nothing more than the assertion of the will and the will remains intact after each act, the individual act of an individual man cannot possibly affect the acts of men as yet unborn.  (b) To disparage the need of Divine grace as a help to man’s weakness through sin. It has been well described as the anthropological side of Arianism in separating man from God.
Although Pelagianism did not issue in any schism, and was perhaps a serious tendency rather than formally a distinct heresy, yet its consequences were absolutely vital to true Christianity.
“It is simply the Christianity of human nature, or that reconstruction of the Gospel scheme which approves itself to natural reason and superficial worldly observation; hence its constant reappearance in the Church.” 
It is true that the Pelagians spoke of grace, but they did not mean by it that supernatural provision in Christ which is intended to meet human sin. The universality of sin was, as our Article suggests, accounted for by Adam’s example and the power of habit, and no corruption of nature even by the growth of habit was allowed.
The teaching of Pelagianism found its antagonist and conqueror in St. Augustine, for when this novel explanation of man’s nature and needs was set forth, it compelled a reconsideration of the entire teaching of Christianity as to human nature and the work of our Lord Jesus Christ. 
Roman Catholic Doctrine
Notwithstanding the efforts of St. Augustine, Pelagianism continued in the form of semi-Pelagianism, and seriously affected the thought of the Middle Ages, and the result was the full Roman Catholic doctrine seen in the sixteenth century. It was taught that original righteousness was not connatural with man, but a superadded gift which, when removed, leaves no detriment behind. The result was that original sin was regarded as the loss of this original righteousness, and the effects of the Fall were simply corporeal, the difference being between a ship in a calm and the same ship in a storm through no fault of the ship. The Council of Trent differs from us in asserting that in Baptism all is removed which is sin, and that though concupiscence remains it is not sin, but is called so because it proceeds from and leads to sin.
This Roman doctrine with all its practical consequences led the Reformers to make definite and strong counter-statements. The Roman Catholic doctrine of “mere nature” was held to be a figment and inconceivable because against experience. The loss of original righteousness was therefore held to be a change involving a corruption of nature. Deprivation must include “depravation.” In opposition to Rome, we add that concupiscence is “of the nature of sin,” meaning as the Article teaches, an infection of nature which is essentially sinful. It has been well remarked, “How the Council could define a thing which is both the effect and the cause of sin not to be in itself sin, or sinful, is not easy to perceive.”  Further, the question of this concupiscence in the unbaptised was not faced by the Council, which was “prudently silent on this point; for it is evident that a thing which is not sin in the baptised, and yet is common to them and the unbaptised, cannot be sin even in the latter.”  It is well to remember that the New Testament deals with sin as a principle before it deals with sins as the aggregation of transgressions or omissions. Following the New Testament in this respect, the Reformers mainly emphasised the depravatio and its source.
The fact is that the Roman Catholic doctrine grew from Pelagianism and was essentially Pelagian in its features. There is no power in nature to enable man to do good, and his greatest need is the grace of God.
>> Part 2. The Extent of Original Sin
 See Skeat, Concise Etymological Dictionary, s.v.
 Article, “Sin,” Hastings’ Dictionary of the Bible; Orr, Sin as a Problem of Today.
 Simpson, Fact and Faith, pp. 107, 101.
 A striking testimony to this truth is seen in the words of Lord Morley, quoted by Dr. Simpson in Fact and Faith (p. 104), in which that statesman, writing an Introduction to a work of Emerson, criticises the American philosopher because he takes no account of “That horrid burden and impediment on the soul, which the Churches call sin, and which, by whatever name we call it, is a very real catastrophe in the moral nature of man” (p. 105).
 Webb, Problems in the Relations of God and Man, p. 118.
 Webb, ut supra, p. 127.
 “This is the core of the whole theory; and all the other postulates not only depend upon it, but arise out of it. Both chronologically and logically this is the root of the system” (Warfield, Two Studies in the History of Doctrine, p. 6).
 “Our life is all of a piece, and the most seemingly isolated actions have both their antecedents and their consequents. The will is not a mere form of choice, which remains unaffected by the actual choices which a man makes; it is affected by them; it gains contents, character, we might almost say nature, from them. If the atomic theory of sin were true – that it consisted only in separate actions – there could be no such thing in man as moral character, either bad or good; for such character is produced by the abiding and cumulative effect of precisely such actions” (Denney, Studies in Theology, p. 81).
Watford (ut supra, p. 10) quotes from Matheson in illumination of the essential nature of Pelagianism:
“Dr. Matheson finely says (Expositor, 1-9, 21) ‘There is the same difference between the Christian and Pagan idea of prayer as there is between the Christian and Pagan idea of sin. Paganism knows nothing of sin, it knows only sins: it has no conception of the principle of evil, it comprehends only a succession of sinful acts.’ This is Pelagianism too.”
 Litton, Introduction to Dogmatic Theology (Second Edition), p. 153.
 For the external history of the Pelagian controversy and of St. Augustine’s part in it, see Warfield, ut supra, pp. 13-139; Bethune Baker, Early History of Christian Doctrine, Ch. 17; Bright, Anti-Pelagian Treatises.
 Litton, ut supra, p. 164, Note 5.
 Litton, ut supra, p. 164.