7 “This parable spoke Jesus unto them; but they understood not what He spoke unto them.” Nor we also, perhaps. What, then, is the difference between them and us, before even we can understand these words? This, that we on our part knock, that it may be opened unto us; while they, by disowning Christ, refused to enter for salvation, and preferred remaining outside to be destroyed. In as far, then, as we listen to these words with a pious mind, in as far as, before we understand them, we believe them to be true and divine, we stand at a great distance from these men.
For when two persons are listening to the words of the gospel, the one impious, the other pious, and some of these are such as neither perhaps understands, the one says, It has said nothing; the other says, It has said the truth, and what it has said is good, but we do not understand it. This latter, because he believes, now knocks, that he may be worthy to have it opened up to him, if he continue knocking; but the other still hears the words, “If you believe not, you shall not understand.” Why do I draw your attention to this?
Even for this reason, that when I have explained as I can these obscure words, or, because of their great abstruseness, I have either myself failed to arrive at an understanding of them, or wanted the faculty of explaining what I do understand, or every one has been so dull as not to follow me, even when I give the explanation, yet should he not despair of himself; but continue in faith, walk on in the way, and hear the apostle saying, “And if in anything ye be otherwise minded, God shall reveal even this unto you. Nevertheless whereto we have already attained, let us walk therein.”
Source: Tractates on the Gospel of John (New Advent)