Why do you think theists follow their religion of choice:
A) Indoctrination of fear at an early age.
B) The promise of reward after death.
C) Everyone else is doing it.
D) The need for spiritual guidance.
E) Because the bible says to.
F) Inability to realize that religion was created by man to control man.
I doubt that people CHOOSE their beliefs. Rather, they are convinced early by their care-givers and then by reason/evidence if they choose to inquire. Of course they can also choose what group to hang out with and they might be convinced by the group propaganda. But part of the indoctrination by care-givers is that they are not to inquire or to hang out with unbelievers...
All of the above. and I would add G) they enjoy the community and fear losing it if they leave, H) (related but different) maintaining both personal and tribal identitiy, I) Status quo bias and J) The enduring effects of hyperactive agency detection.
I will probably think of more later. The point is that religiosity is a pretty complex phenomenon.