I was recently reminded of a physicist, an adult convert to Catholicism, who says he considers Christianity to be a branch of physics. Even more recently, I realized the problem I have with a certain Catholic blogger could be put in these terms: he considers Christianity to be a branch of politics.
From there, I figured I could branch out. Do some Christians consider Christianity to be a branch of psychiatry? Of military science? Of social work?
I wouldn't think many do this sort of thing consciously. But it's easy enough to interpret and even tailor your religious faith according to some other reference that comes to you more naturally (due to temperament or habit, say).
The physicist (something of an anomaly, since he's explicit in his perspective) evidences his own branch theory by trying to find cosmological answers to theological questions. The political branch theorist speaks of Church matters only in political terms (often as not drawing explicit parallels between the Church and the U.S. Congress).
Do I see the Christian Faith as a part, or as a whole?