They were preaching Christ plus something else - rituals, tradition, works. This goes against scripture and Paul goes to great lengths to dispel these notions. Paul emphasises that justification is by faith. Sanctification is also by faith. There is no quick method for sanctification.
Faith is central to the gospel which is proclaimed by faith.
Works do not earn salvation. Many religions advocate this as a necessity for salvation. There is no 'good news'. We can do good by becoming children of God. We do not become children of God by doing good.