6 Thus then He spoke to the Jews, and added, “And He that sent me is with me.” He had already said this also before, but of this important point He is constantly reminding them—“He sent me,” and “He is with me.” If then, O Lord, He is with You, not so much has the One been sent by the other, but you Both have come. And yet, while Both are together, One was sent, the Other was the sender; for incarnation is a sending, and the incarnation itself belongs only to the Son and not to the Father. The Father therefore sent the Son, but did not withdraw from the Son. For it was not that the Father was absent from the place to which He sent the Son. For where is not the Maker of all things? Where is He not, who said, “I fill heaven and earth”? But perhaps the Father is everywhere, and the Son not so? Listen to the evangelist: “He was in this world, and the world was made by Him.” Therefore said He, “He that sent me,” by whose power as Father I am incarnate, “is with me—has not left me.” Why has He not left me? “He has not left me,” He says, “alone; for I do always those things that please Him.” That equality exists always; not from a certain beginning, and then onwards; but without beginning, without end. For Divine generation has no beginning in time, since time itself was created by the Only-begotten.
7. “As He spoke these words, many believed on Him.” Would that, while I speak also, many, who before this were otherwise disposed, understood and believed on Him! For perhaps there are some Arians in this large assembly. I dare not suspect that there are any Sabellians, who say that the Father Himself is one with the Son, seeing that heresy is too old, and has been gradually eviscerated. But that of the Arians seems still to have some movement about it, like that of a putrefying carcass, or certainly, at the most, like a man at the last gasp; and from this some still require deliverance, just as from that other many were delivered. This province, indeed, did not use to have such; but ever since the arrival of many foreigners, some of these have also found their way to our neighborhood. See then, while the Lord spoke these words, many Jews believed on Him. May I see also that, while I am speaking, Arians are believing, not on me, but with me!
8. “Then said the Lord to those Jews who believed on Him, If you continue in my word.” “Continue,” I say, for you are now initiated and have begun to be there. “If you continue,” that is, in the faith which is now begun in you who believe, to what will you attain? See the nature of the beginning, and whither it leads. You have loved the foundation, give heed to the summit, and out of this low condition seek that other elevation. For faith has humility, but knowledge and immortality and eternity possess not lowliness, but loftiness; that is, upraising, all-sufficiency, eternal stability, full freedom from hostile assault, from fear of failure. That which has its beginning in faith is great, but is despised. In a building also the foundation is usually of little account with the unskilled. A large trench is made, and stones are thrown in every way and everywhere. No embellishment, no beauty are apparent there; just as also in the root of a tree there is no appearance of beauty. And yet all that delights you in the tree has sprung from the root. You look at the root and feel no delight: you look at the tree and admire it. Foolish man! What you admire has grown out of that which gave you no delight. The faith of believers seems a thing of little value—you have no scales to weigh it. Hear then to what it attains, and see its greatness: as the Lord Himself says in another place, “If you have faith as a grain of mustard seed.” What is there of less account than that, yet what is there pervaded with greater energy? What more minute, yet what more fervidly expansive? And so “ye” also, He says, “if you continue in my word,” wherein you have believed, to what will you be brought? “you shall be my disciples indeed.” And what does that benefit us? “and you shall know the truth.”
Source: Tractates on the Gospel of John (New Advent)