1. The Article states that the Old Testament “is not contrary to the New.” There is, of course, no question of exact spiritual equality, which has never really been held. The two Testaments are united in all essential features of a progressive revelation without exalting the Old to the spiritual level of the New, and the essential principle is taught by our Lord and His Apostles (Matt. 5:17 f.; John 5:39).
2. The ground of this unity is stated to be the revelation of Jesus Christ as the Messiah. “Both in the Old and New Testament everlasting life is offered to Mankind by Christ, who is the only Mediator between God and Man.” It is because Christ is the subject of both Testaments as the Divine Mediator that we can speak of the vital unity between them (Acts 10:43; Rom. 3:21; Gal. 3:24).
A careful study of the Old Testament will reveal three lines of spiritual teaching. (a) It is a book of unfulfilled prophecies. From the beginning to the end (Gen. 3:15 to Mal. 4:1), while there are prophecies of a temporal and temporary nature which find their fulfilment, the bulk of the announcements refer to the Messiah, and the Old Testament closes with the spirit of expectation. (b) It is also a book of unexplained ceremonies. On almost every page there are references to sacrifices and offerings, and yet there is comparatively little explanation of the meaning of these elements of worship. When the entire organisation of Levitical sacrifices, rites, and ceremonies comes into view the necessity of their explanation becomes more acute, and yet the book closes with little or no real elucidation. (c) It is also a book of unsatisfied longings. From the opening pages to the close there is the frequent expression of desire for God and satisfaction on the part of man. The heart cries out for the Living God and for the blessings God has promised, and though there is great there is no perfect satisfaction, for notwithstanding all the references to the King and the Kingdom, and to God in relation to the spiritual life, as recorded in the Psalms, the book closes in incompleteness (Heb. 7:19). These are the three threads running through it, and they enable us to understand that the Old Testament is almost entirely concerned with the Divine preparation for the redemption of the world; the preparation of the Messiah for the people, and of the people for the Messiah. It is only when we turn to the New Testament that we find the explanation of all this incompleteness. On the very first page we have the keynote, “That it might be fulfilled,” and we are soon able to realise that (a) Jesus Christ the Prophet fulfils (in His life) the prophecies; (b) Jesus Christ the Priest explains (in His death) the ceremonies; and (c) Jesus Christ the King satisfies (in His resurrection) the longings. And so “Jesus, my Prophet, Priest, and King” is the key of the lock, the perfect explanation of the Old Testament and the justification of all its spiritual teaching. Thus, the Article is strictly correct in emphasising the unity and pointing to the ground of this oneness between the two Testaments (Luke 24:27).
>> Part 2. The Spirituality of The Old Testament
 For the topics of this Article see Litton, Introduction to Dogmatic Theology (Second Edition), pp. 44-48.