The Corinthian church's coddling of this perversion gave the people of Corinth the appearance that God's people would allow this sin—a sin even unbelievers would never tolerate! Inside the church it gave the appearance that one could continue in sin and still remain part of the body. The apostle warns them that, just as a pinch of leaven will puff up a whole loaf of bread—or as one rotten apple will corrupt a whole barrel of them—so this sin, if allowed to continue, would ruin the entire church.
Just as a little leaven in bread quickly spreads completely through the dough, one "little" sin affects our whole being. As James points out, if we break one commandment, we are guilty of breaking the whole law. One sin begets another unless the chain is broken through repentance.