He/she believes JW is the truth more than likely because they were indoctrinated from a very young age to believe so.<quoted text>
How do you decide that JW has the truth? If you are making that decision then you are assuming you know the mind of God on some level. Don't you leave any room for doubt?
Do you at least concede that your faith might not be the right one? If not, on what grounds do you have this ultimate Godlike knowledge? And for that matter why are Muslims wrong if they also believe they have the truth? Or Jews or Catholics or Mormons, etc.
There are exceptions, but more often than not a person's religious beliefs are based on their demographic, the culture they live in, their parents' religious beliefs, and the amount of indoctrination they received when children.
If I taught my children the Flying Spaghetti Monster was real from a very young age, basic principal tells me they would believe it. At least (& hopefully) until they came of a age where they can think critically and rationally for themselves.