The nature of GOD, in so far as the mind and affections of man are capable of knowing Him and entering into relationships with Him, is revealed in Jesus Christ His Son, and the revelation is completed and made intelligible by the manifestation of the Holy Spirit. S. Paul expressed the practical content of GOD'S self-disclosure in his phrase "the grace of our Lord Jesus Christ, and the love of GOD, and the fellowship of the Holy Ghost." Later Christian thinkers worked it out into the doctrine of the Holy Trinity, the conception of GOD as at once Three in One, and One in Three. To the plain man the doctrine of the Holy Trinity is something of a puzzle--on the face of it an arithmetical paradox; suggestive, moreover, of the abstract subtleties of speculation rather than of the concrete realities of religious life. But the doctrine did not have its origin, as a matter of historical fact, in any perverse love of subtlety or speculation. It certainly arose out of living realities of spiritual experience. It arose as the result of an attempt, on the part of the earliest Christian believers, to think out the meaning of what had happened in their religious lives, and to express it in speech and thought. What was this thing that had come to them, this thing which had changed their whole outlook upon the world, which had transformed their very inmost souls and made them new men, full of a new vision and a new hope? Something tremendous had happened in their lives. They were confident that it held the secret of _all_ life, for them and for others. It was a new, an overwhelming, a conclusive revelation of GOD. They proclaimed it: they were constrained also to think about it. They had to find ways of expressing it. They had to think out what it meant. There was Jesus Christ. Who was He? What did He mean? What was His relation to man, and to GOD? Certainly He had shed light upon GOD, and upon GOD'S nature. Through His teaching, His character, His life and death, the conception of GOD was filled with a new meaning. In Him GOD was revealed with a fulness that had never been before. He disclosed more of GOD'S inmost character, and more of the relation which He bears to men. "He that hath seen Me hath seen the Father"--the disciples felt that this witness was true. By admitting to their thought of GOD all that the life of Jesus brought, they filled with fresh glory Christ's favourite word for GOD--"your Father which is in Heaven."