So-called extremist islam is the implacable enemy of Christianity, Judaism and secular democracy.
Moderate islam is simply its trojan horse.
Moderate islam is a misnomer, but it is not necessarily a trojan horse. It is decidedly unislamic in its essence, in that it does try to separate faith and day to day life and civil society, but it does exist.
It's a compromise that some Muslims make. If only more of them did....