To reject this message is to choose to be alienated from God. This may not sound Christ like to us. We rightly associate Jesus with love, but remember what Jesus said in Mark 9 about those who lead little ones astray. Harsh language is used about those who would mislead people on crucial matters. Paul repeats this in verse 9.
Different gospels lead to different places because they have different content.
It is news. It is not defined by the messenger. Paul knew his authority was tied to the message he preached. Satan is one of these. Paul knew that he too was not to be the standard of right.