A little leaven (a slight inclination to error, or a few false teachers) leavens the whole lump [it perverts the whole conception of faith or misleads the whole church]. Galatians 5:9 (AMPC)
At one time the disciples of Jesus had forgotten to take bread with them. Jesus told them to be careful and guard against the leaven of the Pharisees and Sadducees. They didn’t understand right away what He meant but as He talked to them farther about what He had done in feeding the multitudes they later discerned that He wasn’t referring to the leaven of bread but the teachings of the Pharisees and Sadducees. Matthew 16:5-12. So a little leaven is the teaching of the law because this is what the Pharisees and Sadducees taught. It’s the teaching of performance based acceptance.
Be careful and guard against any teaching that says you must do something in order for God to accept and help you. God has already totally accepted you in Christ; He has already helped you free of charge in Christ. Simply accept His acceptance and help by believing in Jesus Christ and what He has done on the Cross.