In the previous verse Paul has pronounced God’s curse on all versions of the gospel which differ from the one he first preached to them. Since the gospel is a message of pure grace, then anything which tries to reintroduce a dependence on human achievement or good works must be rejected as another gospel and false.
Christ calls his servants to leave the world and to be set free from its approval. Without this it is impossible to sincerely preach the gospel, for the preacher must condemn the world, and the world will never approve of those who condemn it. Men do not want to hear a message which exposes their sin, which says that the only possible escape from condemnation is to rely on the cross of Christ. The world wants to hear something that gives it some credit, or at least takes the edge off its guilt by making excuses for it. But the preacher has to be fearless in the face of man’s opinion and indifferent to it. While men and women still think there is some righteousness remaining in them, they will never abandon their trust in themselves. They will only reach the point where they are prepared to abandon the ship of good works when they can see it is sinking.
How can a religion flatter its adherents when it sets strict and demanding standards for them? Flattery does not always come in the same form and those who please men can be very subtle in the way they do it. A religion may place many burdens on its followers and criticise them heavily for not conforming to its standards, but fundamentally it is still a religion that flatters. The fact that its approval is hard to gain or is only given to a select few does not alter the fact that those who attain its goals earn the praise of men and are flattered in the process. There may still be a hill to climb but that does not mean that human praise and self-righteousness do not lie at the top of it.