Paul declares that all those who preach a false Gospel of 'faith + works' for salvation are 'anathema' (devoted to destruction), for by adding human works to the finished work of Christ, as being necessary for salvation, they distort and corrupt the true Gospel of Grace,
which is the power of God unto salvation for everyone who believes (Romans 1:16-17).
Thus, they not only deny the completed work of Christ on the Cross, but they also lead their hearers astray into trusting in themselves and their works for salvation, which will inevitably result in their eternal condemnation. We also see how Paul describes his dramatic conversion to Christ, in order to prove that the Gospel that he preached did not come from man, but directly from God through the revelation of Jesus Christ.