It’s a mistake at best and unprofessional at worst.
It’s easy for people for whom religion has brought so much to their lives to want to bring religion to everyone. It may also be the case that the therapist isn’t really equipped to serve someone that’s not at least somewhat spiritual.
How you can tell whether it’s just a mistake or whether it’s unprofessional behavior is the same as getting it to stop. Just ask your therapist why they keep bringing it up and then ask them to stop, because it’s not helping.
If they get defensive, then it’s clearly crossing professional boundaries.