4 For you well know that the Manichees move the unlearned by finding fault with the Catholic Faith, and chiefly by rending in pieces and tearing the Old Testament: and they are utterly ignorant, how far these things are to be taken, and how drawn out they descend with profit into the veins and marrows of souls as yet as it were but able to cry. And because there are in them certain things which are some slight offense to minds ignorant and careless of themselves, (and there are very many such,) they admit of being accused in a popular way: but defended in a popular way they cannot be, by any great number of persons, by reason of the mysteries that are contained in them.
But the few, who know how to do this, do not love public and much talked of controversies and disputes: and on this account are very little known, save to such as are most earnest in seeking them out. Concerning then this rashness of the Manichees, whereby they find fault with the Old Testament and the Catholic Faith, listen, I entreat you, to the considerations which move me. But I desire and hope that you will receive them in the same spirit in which I say them. For God, unto Whom are known the secrets of my conscience knows, that in this discourse I am doing nothing of evil craft; but, as I think it should be received, for the sake of proving the truth, for which one thing we have now long ago determined to live; and with incredible anxiety, lest it may have been most easy for me to err with you, but most difficult, to use no harder term, to hold the right way with you.
But I venture to anticipate that, in this hope, wherein I hope that you will hold with us the way of wisdom, He will not fail me, unto Whom I have been consecrated; Whom day and night I endeavor to gaze upon: and since, by reason of my sins, and by reason of past habit, having the eye of the mind wounded by strokes of feeble opinions, I know that I am without strength, I often entreat with tears, and as, after long blindness and darkness the eyes being hardly opened, and as yet, by frequent throbbing and turning away, refusing the light which yet they long after; specially if one endeavor to show to them the very sun; so it has now befallen me, who do not deny that there is a certain unspeakable and singular good of the soul, which the mind sees; and who with tears and groaning confess that I am not yet worthy of it.
He will not then fail me, if I feign nothing, if I am led by duty, if I love truth, if I esteem friendship, if I fear much lest you be deceived.
Source: On the Profit of Believing (New Advent)