50 But, again, lest by occasion of this sentence, any one should sin with deadly security, and should allow himself to be carried away, as though his sins were soon by easy confession to be blotted out, he straightway added, “My little children, these things have I written unto you, that you sin not; and, if one shall have sinned, we have an Advocate with the Father, Jesus Christ the righteous, and Himself is a propitiation of our sins.” Let no one therefore depart from sin as though about to return to it, nor bind himself as it were by compact of alliance of this kind with unrighteousness, so as to take delight rather to confess it than to shun it. But, forasmuch as even upon such as are busy and on the watch not to sin, there creep by stealth, in a certain way, from human weakness, sins, however small, however few, yet not none; these same themselves become great and grievous, in case pride shall have added to them increase and weight: but by the Priest, Whom we have in the heavens, if by pious humility they be destroyed, they are with all ease cleansed.
Source: On Holy Virginity (New Advent)