“If you deviate from the gospel in what you say about the gospel, or if you deviate from the gospel in how you act, you’re in violation of the purity of the gospel. It’s hard; I understand. It’s hard to be bold for the gospel when you’re with people who compromise the gospel but also talk about Christ. It’s hard to talk to someone in a form of Christianity that is apostate, heretical, outside the bounds of the true gospel. It’s hard to talk to a Roman Catholic or somebody in some cult or some fringe group, or any kind of ‘Christian’ organization that has a review of the gospel that’s in error.
“It’s hard to be bold, because you want to be accepted by them. And maybe you say, “Well, you know, they’re not going to listen to what I have to say if they don’t” – it’s hard. And the fear of men brings a snare, doesn’t it? Even Peter had to be confronted to the face.
Don’t attack the gospel. Don’t attack it by changing it in its content, and don’t attack it by siding with people who have a false gospel. You can’t do that. Paul is saying to the church at Galatia, ‘We have to have the gospel clear; that’s why we’re in the world.'”
— John MacArthur, The Danger Of Adding To The Gospel