In (the) church

In my opinion, "the" is not required if the church in question is the one that Tom regularly goes to (to attend a service). Otherwise, either "the" or "a" is needed.

I thought that if I say "the" church I'm referring to the one Tom always goes to, only to this one, particular church. And if I said "a" church then in my view it would mean that it was any church out of ten churches which you can go to in some particular city.

Thanks

When you say "Tom's in church", you mean he's there for the service. If he or anyone else passes out during a service, then I think you can still use "in church" rather than "in the church". But if Tom were a handyman doing some work in the church, then you would probably say "he passed out in the church".

This issue is highly idiomatic so you may want to hear from native speakers as well