I'm currently reading through, with great profit, Phil Ryken's commentary on Galatians in the Reformed Expository Commentary series. Here's a snippet:
We cannot simply assume that we have the gospel. Unless we keep the gospel at the center of the church, we are always in danger of shoving it off to one side and letting something else take its place.Ryken, Galatians, p. 21
Martin Luther rightly warned that "there is a clear and present danger that the devil may take away from us the pure doctrine of faith and may substitute for it the doctrines of works and of human traditions..." The good news of the cross and resurrection must be preached, believed, and lived. Otherwise it will be lost.
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 that 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."