Galatians 1:6-9 I am astonished that you are so quickly deserting him who called you in the grace of Christ and are turning to a different gospel— not that there is another one, but there are some who trouble you and want to distort the gospel of Christ. But even if we or an angel from heaven should preach to you a gospel contrary to the one we preached to you, let him be accursed. As we have said before, so now I say again: If anyone is preaching to you a gospel contrary to the one you received, let him be accursed.
In Philip Ryken's expository commentary, his comments about this passage are very helpful. In the brief snippet below he is addressing the way the Judaizers-the ones troubling the Galatians by distorting the gospel - were adding works (circumcision) to the gospel rendering it no gospel at all. Ryken warns-
The church's greatest danger is not the anti-gospel outside the church; it is the counterfeit gospel inside the church. The Judaizers did not walk around Pisidian Antioch wearing T-shirts that said, "Hug me, I'm a false apostle." What made them so dangerous was that they knew how to talk the way Christians talk. They used all the right terminology. They talked about how they "got saved." They told people to "trust in Christ." They "presented the gospel."
Only they did not have the gospel after all. We should expect, therefore, that the most serious threat to the one true gospel is something that is also called the gospel. The most dangerous teachers are the ones who preach a different Christ but still call him "Jesus".