Actually, in the Campaign Guide it does specifically state that clerics, druids, rangers and paladins MUST worship a deity (and the concept is that in the Realms all divine casters must 'commit') and that the option to worship a 'concept' is not allowed in FR.
If your character is not
one of those classes your not required to worship a deity and if you don't then yes, returning from the dead becomes problematic.
Playing a priest of Kelemvor makes for interesting philosophical conversations with the groups 'in game atheist', which is why I am positive on this. So far that atheist has 'died' three times and each time some magic was used to prevent the actual death, with revivify doing it just last night after a failed save to Wail of the Banshee, so she has never had to face the decision of commiting to a deity and coming back or become building material for the Fugue plane.