3 But to the Lord's own answer, let us give better and more earnest heed, lest we ourselves be also found bondmen. For “Jesus answered them, Verily, verily, I say unto you, that every one who commits sin is the servant of sin.” He is the servant— would that it were of man, and not of sin! Who will not tremble at such words? The Lord our God grant us, that is, both you and me, that I may speak in fitting terms of this freedom to be sought, and of that bondage to be avoided.
“Amen, amen [verily, verily], I say unto you.” The Truth speaks: and in what sense does the Lord our God claim it as His to say, “Amen, amen, I say unto you”? His charge is weighty in so announcing it. In some sort, if lawful to be said, His form of swearing is, “Amen, amen, I say unto you.” Amen in a way may be interpreted, [It is] true [truly, verily]; and yet it is not interpreted, though it might have been said, What is true [verily] I say unto you. Neither the Greek translator nor the Latin has dared to do so; for this word Amen is neither Greek nor Latin, but Hebrew.
So it has remained without interpretation, to possess honor as the covering of something hidden; not in order to be disowned, but that it might not, as a thing laid bare to the eye, fall into disrepute. And yet it is not once, but twice uttered by the Lord, “Amen, amen, I say unto you.” And now learn from the very doubling, how much was implied in the charge before us.
Source: Tractates on the Gospel of John (New Advent)