Please remember that you're Generalizing from Fictional Evidence.

Human beings are not perfect liars. A primary problem is that we are rather well evolved to detect deception from other human beings, especially our mates; there is no reason to believe he could hide his deconversion from his friends or his wife. Another problem is that humans tend to slowly believe the lies we tell. Would you consider it a beneficial side effect if he accidentally reconverted to Christianity due to this deception?

There is a cost/benefit for every lie we tell. Generally it turns out that honesty really is the best policy. This case seems to be no different.