12 Although therefore all our sins were forgiven in the “laver of regeneration,” we should be driven into great straits, if there were not given to us the daily cleansing of the Holy Prayer. Alms and prayers purge away sins; only let not such sins be committed, for which we must necessarily be separated from our daily Bread; avoid we all such debts to which a severe and certain condemnation is due. Call not yourselves righteous, as though ye had no cause to say, “Forgive us our debts, as we also forgive our debtors.”
Though you abstain from idolatry, from the consolations of astrologers, from the cures of enchanters, though ye abstain from the seductions of heretics, from the divisions of schismatics; though ye abstain from murders, from adulteries and fornications, from thefts and plunderings, from false witnessings, and all such other sins which I do not name, as have a ruinous consequence, for which it is necessary that the sinner be cut off from the altar, and be so bound in earth, as to be bound in heaven, to his great and deadly danger, unless again he be so loosed in earth, as to be loosed in heaven; yet after all these are excepted, still there is no want of occasions whereby a man may sin.
A man sins in seeing with pleasure what he ought not to see. Yet who can hold in the quickness of the eye? For from this the eye is said to have received its very name, from its quickness. Who can restrain the ear or eye? The eyes may be shut when you will, and are shut in a moment, but the ears you can only with an effort close: you must raise the hand and reach them, and if any one hold your hand, they are kept open, nor can you close them against reviling, impure, or flattering, and seducing words.
And when you hear any things you ought not to hear, though you do it not, do you not sin with the ear? For you hear something that is bad with pleasure? How great sins does the deadly tongue commit! Yea, sometimes sins of such a nature, that a man is separated from the altar for them. To the tongue pertains the whole matter of blasphemies, and many idle words again are spoken, which are not convenient. But let the hand do nothing wrong, let the feet run not to any evil, nor the eye be directed to immodesty; let not the ear be open with pleasure to filthy talk; nor the tongue move to indecent speech; yet tell me, who can restrain the thoughts?
How often do we pray, my brethren, and our thoughts are elsewhere, as though we forgot before whom we are standing, or before whom we are prostrating ourselves! If all these things be collected together against us, will they not therefore not overwhelm us, because they are small faults? What matter is it whether lead or sand overwhelm us? The lead is all one mass, the sand is small grains, but by their great number they overwhelm you. So your sins are small. Do you see not how the rivers are filled, and the lands are wasted by small drops? They are small, but they are many.
Source: Sermons on Selected Lessons of the New Testament (New Advent)