From my studies last week, I found this wonderful explanation of the Trinity and how we come to find it in Scripture.
“‘Is the doctrine of the Trinity revealed in the Old Testament, or is it merely revealed in the New Testament?’ Strange as it may seem, it is not exactly correct to say that it is revealed in either. As Dr. B.B. Warfield once said, ‘We cannot speak of the doctrine of the Trinity… if we study exactness of speech, as revealed in the New Testament, any more than we can speak of it as revealed in the Old Testament. The Old Testament was written before its revelation; the New Testament after it. The revelation itself was made not in word but in deed. It was made in the incarnation of God the Son, and the outpouring of God the Holy Spirit. The relation of the two Testaments to this revelation is in the one case that of preparation for it, and in the other that of product of it.”’
“God revealed himself by supernatural deeds, along with which he gradually gave more and more verbal interpretation. Only as God’s plan of redemption was fully worked out, was God himself fully made known. God could have announced at the very beginning that there was with the unity of his being three distinct Persons. But who could have understood? But when, in the fullness of time, each of the three Persons actually wrought before the eyes of men those mighty deeds of redemption which each Person of the God-head was to do in the plan of salvation, who could not understand? Thus in Scripture we have the record of that which God of the doctrine the Trinity is supplied in the recorded fact that the Father manifestly is God, that Jesus just as clearly is God and so also, the Holy Spirit.”
 G.I. Williamson, The Westminster Confession of Faith For Study Classes, Presbyterian and Reformed Publishing Co., Philadelphia, PA, 1964, pp. 26-27.