It is true of ideas, as of men, that they cannot fight unless they occupy the same ground: ideas that rush toward each other on different levels of apprehension will pass without conflict or mutual injury because they never establish contact, never collide.
~ Carl Becker, The Heavenly City of the 18th-Century Philosophers
This is perhaps not what Blake had in mind when he wrote that “opposition is true friendship,” but the phrase comes to mind. The outright infidel is always a stranger, reasoning from alien assumptions to alien conclusions. You are a ghost to his knife. The heretic, however, is always dear – a brother, child, friend – and draws blood.