He is focusing on the problem of the merger of Evangelical faith with political ideology. The idea of a Christian Nation is one of the "master signifiers," in his view, that serves as a unifying symbol for the conservative Evangelical political ideology. (Fitch is theologically conservative). His overall point is that by tying Evangelical faith to a political ideology (and even a political party) which is now in trouble in the public sphere, as the ideology falls on hard times, Evangelical faith falls with it. Especially among the emerging generation of young adults.
I hope this does not sound too harsh, but this particular issue reminds of Jesus' words in Matthew 23:
13 “I’ve had it with you! You’re hopeless, you religion scholars, you Pharisees! Frauds! Your lives are roadblocks to God’s kingdom. You refuse to enter, and won’t let anyone else in either.