My sulky student brother was blackmailed into black jeans and a black shirt
for the occasion by my mother, and he was mistaken for the new curate by
half the parish !
and guessing the Christian Church in question is most likely Roman Catholic and not some esoteric sect with a dress code of robes for members, the only possible answer is a suit (dark coloured, preferably navy or grey, leave pinstripes, herringbone etc for the office, modest solid colour tie, shirt and polished shoes). You could also wear the same to your son’s christening
I am not a practicing believer of any religion, but when having occasion to attend religious services from time to time I would make a point of donning suit and tie or at least a sports jacket and tie. By doing so I am showing respect and courtesy towards the occasion and the beliefs of those attending the service.
Even if jeans, runners, hoodies for guys and crop tops (to better show of body piercings and tattoos) may be the rapidly approaching norm for Chruch goes I choose not to follow the masses (no pun intended).
... or at least a sports jacket and tie. By doing so I am showing respect and courtesy towards the occasion and the beliefs of those attending the service.