Galatians 5:9 A little leaven leavens the whole lump.
12 It is those who want to make a good showing in the flesh who would force you to be circumcised, and only in order that they may not be persecuted for the cross of Christ.
13 For even those who are circumcised do not themselves keep the law, but they desire to have you circumcised that they may boast in your flesh.
Luke 12:1 In the meantime, when so many thousands of the people had gathered together that they were trampling one another, he began to say to his disciples first, "Beware of the leaven of the Pharisees, which is hypocrisy. (Luke 18:9-14)
In the context of Galatians 5 (indeed the entirety of the letter), the leaven that Paul refers to is not sin but legalism, which of course leads to all kinds of non-biblical behavior. These enforcers of the law boast in themselves i.e. in their own piety and that of their "disciples". They teach their followers to do the same. We see this as much today as it has ever been seen; deliverance ministries, the "doctrine' of sinless perfection, those who teach salvation can be lost, progressive sanctification teachings, and on and on. Evidently Christ has not done it all.
Grace is a stumbling block to the legalists, the judaizers, and the purveyors of pious blether.
Thank God it is never left up to us!
30 And because of him you are in Christ Jesus, who became to us wisdom from God, righteousness and sanctification and redemption,
31 so that, as it is written, "Let the one who boasts, boast in the Lord."