4 This issue arose because some false brothers had come in under false pretenses to spy on our freedom in Christ Jesus, in order to enslave us. -Galatians 2:4 BSB
The phrase “This issue” is about making believers go back to the Law of Moses to observe it and keep it. See verse 3 and its commentary.
What makes “some false brothers” is when they add to God’s Word, which changes it to say something other than what God said, and preaches it to others as the truth.
False teachers came to the Galatians and pretended to believe in sound teachings like salvation by grace through faith in Jesus Christ (Ephesians 2:8-9), not revealing to them that, in reality, they hold to work-based salvation. They teach that a person needs to keep the Law, which is works, plus believe in Jesus Christ to get saved. Read Galatians 3:1-2.
Paul understood that Satan is the one who influences false teachers, see 2 Corinthians 11:13-15, to enslave believers by entrapping them, teaching them to keep the Law, which makes them forsake the finished work of the cross, see Galatians 5:1-4. The cross—Jesus, who died for our sins (John 3:16, Romans 5:6-9), was to show us that our good works cannot atone for our sins and make us right in God’s eyes (Galatians 3:11). Only by accepting the gift that God offers through His Son, the Lord Jesus Christ, into our hearts by faith in Him can we have all of our sins forgiven.