29 But by these very words He proclaims Himself plainly to be true God in the nature of the only true God. To understand this, let our answer proceed from statements which He made previously, though the connection is unbroken right down to these words. We can then establish the faith step by step, and let the confidence of our freedom rest at last on the summit of our argument, the true Godhead of Christ. There comes first the mystery of His words, He that has seen Me, has seen the Father; and, Do ye not believe Me that I am in the Father and the Father in Me?
The words that I say unto you, I speak not from Myself; but the Father abiding in Me, Himself does His works. Believe Me that I am in the Father and the Father in Me: or else believe Me for the very works' sake. At the close of this discourse, teeming with deep mysteries, follows the reply of the disciples, Now know we that You know all things, and needest not that any man should ask you: by this we believe that You came forth from God. They perceived in Him the nature of God by the divine powers which He exercised; for to know all things, and to read the thoughts of the heart belongs to the Son, not to the mere messenger of God. They confessed, therefore, that He had come from God, because the power of the divine nature was in Him.
Source: On the Trinity (New Advent)