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)