What does 'a little leaven leavens the whole lump' mean?

The phrase 'a little leaven leavens the whole lump' (Galatians 5:9) illustrates how even minor additions of works or legalism can fundamentally corrupt the message of the Gospel. Leaven, in this context, represents sin or false doctrine. When legalistic ideas are introduced, they can spread throughout and influence the entire understanding of salvation, leading people away from reliance on grace alone. Paul uses this analogy to caution the Galatian believers against the false teachings that detracted from the truth of the Gospel. It emphasizes the need for vigilance in maintaining the purity of the message of grace without any works or merit added, as it is God's grace that saves, not our efforts.