No, it's not a slur. All Christians are not evangelicals. There are mainline churches, there are Catholic and orthodox churches, then there are evangelicals.
Definition: belonging to or designating the Christian churches that emphasize the teachings and authority of the Scriptures, especially of the New Testament, in opposition to the institutional authority of the church itself, and that stress as paramount the tenet that salvation is achieved by personal conversion to faith in the atonement of Christ.
That's my reading, anyway.
If I'm wrong, God will 'splain me. I imagine that's pretty far down on my list of 'of sh--' stuff, though.