Upon the discourses delivered yesterday and the day before, follows the Gospel lesson of today, which we must endeavor to expound in due course, not indeed proportionably to its importance, but according to our ability: both because you take in, not according to the bountifulness of the gushing fountain, but according to your moderate capacity; and we too speak into your ears, not so much as the fountain gives forth, but so much as we are able to take in we convey into your minds,— the matter itself working more fruitfully in your hearts than we in your ears.
For a great matter is treated of, not by great masters, nay, rather by very small; but He who, being great, for our sakes became small, gives us hope and confidence. For if we were not encouraged by Him, and invited to understand Him; if He abandoned us as contemptible, since we were not able to partake His divinity if He did not partake our mortality and come to us to speak His gospel to us; if He had not willed to partake with us what in us is abject and most small—then we might think that He who took on Himself our smallness, had not been willing to bestow on us His own greatness.
This I have said lest any should blame us as over-bold in handling these matters, or despair of himself that he should be able to understand, by God's gift, what the Son of God has deigned to speak to him. Therefore what He has deigned to speak to us, we ought to believe that He meant us to understand. But if we do not understand, He, being asked, gives understanding, who gave His Word unasked.
Source: Tractates on the Gospel of John (New Advent)