There's no formal distinction. For centuries, the most prominent philosophers were of course religious (since it would be heretical not to be).

Some of the most important philosophers were strongly Christian and their religion was a fundamental part of their philosophy. E.g. Descartes (and his sketchy ontological argument for God) and Kant (and his belief in God as the only explanation for morality) are two of the most obvious examples that spring to mind.

Atheists in philosophy only became prominent when atheism was tolerated in society. Hume was one of the first examples in the 18th century - never an overt atheist, but considered an atheist at the time, and therefore a receiver of social scorn. Jeremy Bentham in the early 19th century was a fierce critic of religion. Overt atheism in philosophy wasn't socially acceptable until the late 19th century / early 20th, and when we saw books like Bertrand Russell's Why I Am Not A Christian in the 1920s.

Basically, religion is an essential theme that has run throughout the course of the history of philosophy. The commenter who said "most fields are entirely agnostic to religious positions" is flat out wrong. Although he's probably just talking about very recent philosophy, for which it would be true. But when looking at the entire history of philosophy, religion is one of the core strands. It was the underpinning of all human thought for centuries / millennia.