10 “If I do not the works of my Father, believe me not. But if I do, though ye will not believe me, believe the works; that you may know and believe that the Father is in me, and I in Him.” The Son says not, “the Father is in me, and I in Him,” as men can say it. For if we think well, we are in God; and if we live well, God is in us: believers, by participating in His grace, and being illuminated by Himself, are in Him, and He in us. But not so is it with the only-begotten Son: He is in the Father, and the Father in Him; as one who is equal is in him whose equal he is. In short, we can sometimes say, We are in God, and God is in us; but can we say, I and God are one? You are in God, because God contains you; God is in you, because you have become the temple of God: but because you are in God, and God is in you, can you say, He that sees me sees God; as the Only-begotten said, “He that has seen me, has seen the Father also;” and “I and the Father are one”? Recognize the prerogative of the Lord, and the privilege of the servant. The prerogative of the Lord is equality with the Father: the privilege of the servant is fellowship with the Saviour.
Source: Tractates on the Gospel of John (New Advent)