It has been a long-standing custom in H&S circles, established by the HSE, that (unpaid) volunteers should be treated in the same way as (paid) employees. That is, they should be given the appropriate training, protective clothing, etc. that any paid employee should be given for the job they are doing. I see no reason why this should be different for specific fire safety matters.
In churches the problem arises not just about volunteers doing jobs round the premises, but the congregation as a whole, both those who regularly attend services and those who are there for baptisms, weddings and funerals. In the former case they probably know their way out, in the latter case you may need to organise regular members to act as stewards, and they will certainly will need training on evacuation procedure and the like.