Here in the book of Galatians, we see that Paul uses very strong language to express himself because the church at Galatia had departed from the gospel of grace and had turned to a gospel of works to maintain their salvation. They seemed to think that even though they were saved by grace that they had to "prove" their salvation by doing some type of work (i.e. circumcision). Today, it would appear that water baptism is that work. How many religious denominations will allow someone to become a member of their fellowship without water baptism of some sort?
Paul tells us that when we add works, we are perverting the gospel of the grace of God and that perversion then becomes another gospel. Paul says that anyone who preaches another gospel, other than what he preached, that person is accursed. What Christ accomplished on the cross was sufficient to satisfy the justice of God; there is nothing that we can do to gain or maintain our salvation.
Paul stresses the fact that the gospel he preached was not received from man but by revelation of Jesus Christ.
(Galatians 2:16 KJV) Knowing that a man is not justified by the works of the law, but by the faith of Jesus Christ, even we have believed in Jesus Christ, that we might be justified by the faith of Christ, and not by the works of the law: for by the works of the law shall no flesh be justified.