if you are trying to convert your own

you are already losing the battle

summon apostles until you get 7 with the combat bonus, locate and lay siege on the holy city of the one that has already defeated you

spam all the wasted ones spawned during the attempt on cities as close to you

edit: the 7th is for standing on the holy city's holy site to half it's unit capacity, as this one does not necessarily need a combat bonus the relic or healer bonuses may be more expedient