Augustine of Hippo

And lest haply he should seem to have given impunity for sins, in that he said, "He is faithful and just to cleanse us from all iniquity;" and men henceforth should say to themselves, Let us sin, let us do securely what we will, Christ purgeth us, is faithful and just, purgeth us from all iniquity: He taketh from thee an evil security, and putteth in an useful fear. To thine own hurt thou wouldest be secure; thou must be solicitous. For "He is faithful and just to forgive us our sins," provided thou always displease thyself, and be changing until thou be perfected. Accordingly, what follows? "My little children, these things I write unto you, that ye sin not." But perchance sin overtakes us from our mortal life: what shall be done then? What shall there be now despair? Hear: "And if any man sin, we have an advocate with the Father, Jesus Christ the righteous: and He is the propitiator for our sins." He then is the advocate; do thou thine endeavor not to sin: if from the infirmity of this life sin shall overtake thee, see to it straightway, straightway be displeased, straightway condemn it; and when thou hast condemned, thou shall come assured unto the Judge. There hast thou the advocate: fear not to lose thy cause in thy confession. For if oft-times in this life a man commits his cause to an eloquent tongue, and is not lost; thou committest thyself to the Word, and shall thou be lost? Cry, "We have an advocate with the Father."