1 Corinthians 5:6

Then, having ended his sentence, and spoken it in brief without dwelling on it, he brings in again a rebuke, directing himself against them; "Your glorying is not good:" signifying that it was they up to the present time who had hindered him from repenting, by taking pride in him. Next he shews that he is taking this step in order to spare not that person only, but also those to whom he writes. To which effect he adds, "Know ye not, that a little leaven leaveneth the whole lump?" "For," saith he, "though the offence be his, yet if neglected it hath power to waste the rest of the body of the Church also. For when the first transgressor escapes punishment, speedily will others also commit the same faults." In these words he indicates moreover that their struggle and their danger is for the whole Church, not for any one person. For which purpose he needeth also the similitude of the leaven. For "as that," saith he, "though it be but little, transforms unto its own nature the whole lump; so also this man, if he be let go unpunished and this sin turn out unavenged, will corrupt likewise all the rest."