9 What, brethren, does He promise believers? “And you shall know the truth.” Why so? Had they not come to such knowledge when the Lord was speaking? If they had not, how did they believe? They believed, not because they knew, but that they might come to know. For we believe in order that we may know, we do not know in order that we may believe. For what we shall yet know, neither eye has seen, nor ear heard, nor has it entered the heart of man. For what is faith, but believing what you see not?
Faith then is to believe what you see not; truth, to see what you have believed, as He Himself says in a certain place. The Lord then walked on earth, first of all, for the creation of faith. He was man, He was made in a low condition. He was seen by all, but not by all was He known. By many was He rejected, by the multitude was He slain, by few was He mourned; and yet even by those who mourned Him, His true being was still unrecognized. All this is the beginning as it were of faith's lineaments and future up-building.
As the Lord, referring thereto, says in a certain place, “He that loves me keeps my commandments; and he that loves me shall be loved of my Father, and I will love him, and will manifest myself to him.” They certainly already saw the person to whom they were listening; and yet to them, if they loved Him, does He give it as a promise that they should see Him. So also here, “You shall know the truth.” How so? Is that not the truth which You have been speaking? The truth it is, but as yet it is only believed, not beheld.
If you abide in that which is believed, you shall attain to that which is seen. Hence John himself, the holy evangelist, says in his epistle, “Dearly beloved, we are the sons of God; but it is not yet apparent what we shall be.” We are so already, and something we shall be. What more shall we be than we are? Listen: “It is not yet apparent what we shall be: [but] we know that, when He shall appear, we shall be like Him.” How? “For we shall see Him as He is.” A great promise, but the reward of faith.
You seek the reward; then let the work precede. If you believe, ask for the reward of faith; but if you believe not, with what face can you seek the reward of faith? “If” then “ye continue in my word, you shall be my disciples indeed,” that you may behold the very truth as it is, not through sounding words, but in dazzling light, wherewith He shall satisfy us: as we read in the psalm, “The light of Your countenance is impressed upon us.” We are God's money: we have wandered away as coin from the treasury.
The impression that was stamped upon us has been rubbed out by our wandering. He has come to refashion, for He it was that fashioned us at first; and He is Himself asking for His money, as Cæsar for his. Therefore He says, “Render unto Cæsar the things that are Cæsar's, and unto God the things that are God's:” to Cæsar his money, to God yourselves. And then shall the truth be reproduced in us.
Source: Tractates on the Gospel of John (New Advent)