People who are deeply religious and have been so for a long time (and especially if they have been since childhood) often have a really hard time letting go if they start having doubts and thinking about these things more rationally.
There is, of course, a multitude of reasons for this. Religious dogmas contain tons and tons of "self-protection mechanisms", ie. notions and ideas that exist to make it psychologically as hard as possible for the deeply religious to discard the religion. These mechanism are what make religions so prevalent and pervasive.
Anyways, I have been wondering if at least in some cases one aspect that's strongly in play is a concept of "sunk cost fallacy." This is a fallacy that's most often applied to economics: If someone has invested a lot of money in a project that turns out to be a fiasco that never gets finished with a satisfactory result, this person will often have a strong aversion to just stop funding it because the idea of just outright losing all that money is unbearable. It's better, in their minds, to spend just a little more in order to get something sellable out of the project than simply dropping it and accepting a pure loss of money. It's better to get even some of the money back from a half-finished project than nothing at all. In the worst cases such people have sunk vast amounts of money on hopeless projects. It can become a vicious cycle: The more money is spent, the less motivation to just end it (because the total losses would be very large.)
When speaking about religion the question is not necessarily about money, but about time, effort, work and principles. It may well be that someone who has been deeply religious for tens of years might at some level, even without realizing it, think along the lines of "I have spent so much time in this, so much effort, studied the Bible so much, invested so much emotionally, made so many friends, that I cannot just outright stop now. Even the idea of stopping now and throwing all that away is unbearable. Out of the question." They might not literally think like that, but that very notion may well be one of the driving forces behind them keeping their religious beliefs. The very idea that they might have spent so much time and effort on a lie is so unbearable that they don't even want to consider it.