Well, he did say it!

Religion and politics should not mix. They are two separate things. Faith/religion is a personal thing, and should remain separate from the public political sphere. Whether a politician is a member of any faith/has any religious beliefs doesn’t make any difference as to whether they can be a politician, so long as they place a divide between their faith and their political actions. Have beliefs, but don’t preach them. Religion should stay out of politics. It has no place in it. Britain is, in practice, a secular society and this should be reflected in our politicians.

The juxtaposition of this with American politics is striking, especially considering that in America there is an official separation between the two

Sources: The Telegraph, BBC