| Preach to the choir|
|Crux of the matter|
|Speak of the devil|
|An act of faith|
Gateway religion is the idea that moderate or liberal religions (and sects within religions) ultimately act as a "gateway" to irrational thinking, regardless of how palatable their politics may be. It's mostly used by antitheists (notably Richard Dawkins in The God Delusion) to argue against the acceptance of any religion.
The evidence for this being a real effect is varying, relying almost entirely on anecdotes — some assert the opposite, that people tend to flip between extremism or fundamentalism and atheism, completely skipping the moderate step in between. Other assertions are that the existence and tolerance of moderate religious belief acts as a gateway that allows extremism to be routinely tolerated.