Seems trivially true, to me. The european feudal system developed out of the late roman imperial system, that would have vicars and dukes, with the dukes representing secular military authority.

Also, off the top of my head, I think somebody like Charlemagne (as the first guy I know who merged military rule with the support of the pope) would have been wholy interested in christian support for secular reasons (and vice versa). Being the roman emperor, appointed by the pope, would presumably help with his legitimacy in Italy and Gaul.