Costly religious displays evolved as honest signals of group commitment.

Evidence

Similar to these animal signals, some theorists see extravagant religious displays as costly signals of group commitment that was a naturally selected adaptation for life in cooperative groups. In this view, a signal is reliable only to the extent that it is costlier to fake by potential freeloaders than for cooperators. If religious groups are cooperative groups, what would prevent selfish imposters from faking prosocial intent, receiving cooperative benefits without reciprocating? Because mere professions of religious belief can be easily faked, evolutionary pressures have favored costly religious displays that are not subject to rational calculations of cost-benefit analysis. Thus, costly religious behaviors are seen as naturally selected honest signals that reliably advertise the unobservable trait of group commitment. — Norenzayan, Ara, Big gods - how religion transformed…, 340

Categorization

• XREF: The causal logic here — cheap professions of belief are faked easily, so costly acts are required to signal sincerity — maps onto why merely ordering a priest to deny Christ privately would fail; the Japanese must force a public, costly, observable act (trampling the fumi-e) to strip the partner-choice value of his faith.