5 We did not give in to them for a moment, so that the truth of the gospel would remain with you. -Galatians 2:5 BSB
Paul told the saints in Galatia that if he and his companions did not resist them and fought for the truth of the gospel, they (the church in Galatia) would have never received and continued with the true gospel of Jesus Christ.
This verse shows us that we can never give in to false doctrine or allow false believers to spread their unbiblical message to others if we can help it. Especially if you become aware that their deceptive message is reaching those you are trying to influence to follow Christ and walk in the truth. If you don’t battle false teachers, they will undo everything you had begun to do for the Lord Jesus Christ.