Leaven.

Galatians 5:2-10
2 Indeed I, Paul, say to you that if you become circumcised, Christ will profit you nothing. 3 And I testify again to every man who becomes circumcised that he is a debtor to keep the whole law. 4 You have become estranged from Christ, you who attempt to be justified by law; you have fallen from grace. 5 For we through the Spirit eagerly wait for the hope of righteousness by faith. 6 For in Christ Jesus neither circumcision nor uncircumcision avails anything, but faith working through love.     7 You ran well. Who hindered you from obeying the truth? 8 This persuasion does not come from Him who calls you. 9 A little leaven leavens the whole lump. 10 I have confidence in you, in the Lord, that you will have no other mind; but he who troubles you shall bear his judgment, whoever he is.