adopt_religion_interaction_effect

adopt_religion_interaction_effect = {
	# Convert Capital (if of same faith as recipient's old faith)
	if = {
		limit = {
			scope:actor = {
				highest_held_title_tier > tier_barony
				capital_county.faith != scope:recipient.faith
			}
		}
		scope:actor.capital_county = {
			set_county_faith = scope:recipient.faith
			set_county_rite = scope:recipient.rite
		}
	}

	# Spouses and family adopt new faith
	grab_spouses_and_family_to_convert_effect = yes
	convert_family_to_faith_effect = {
		FALSE_CONVERSION = no
		FORCED = no
	}
}

As written in game/common/scripted_effects/00_religious_interaction_effects.txt.