The Galatian believers had become fascinated by the false teachers arguments, almost as thou they had been bewitched. Magic was common in the apostle Paul day. Magicians used both illusions & Satan's power to perform miracles.
The believers in Galatia, many of whom may have been in Jerusalem at Pentecost & received the Holy Spirit there, knew they didn't receive God's Spirit by obeying the Jewish laws. We must realize that we grow spiritually because of God's work in us, not by following special rules.
The Galatians knew they received the Holy Spirit when they believed, not when they obeyed the Law.
The Holy Spirit gives Christians great power to live for God. Some Christians want more than this.
Reference summary used from the KJV Life Application Bible, Tyndale House Publishers, Inc., Wheaton, Illinois