Many will turn from the faith

I have something on my mind that has been bothering me, so my grandad was a great Christian, he knows the Bible inside out and he lead a very Godly life when my mom was being raised. When him and my grandma got divorced it crushed him, he never imagined it would happen, he went through many years of depression feeling as if God had abandoned him, some would call this the dark night of the soul. He met a trans person and I think that changed him, he secretly always liked both men and woman and when he got the chance he took it, after that he became extremely liberal, he became a pot head like in his youth, started having sex with transgendered people, and he now absolutely despises God, he does everything he can to disprove the Bible even tho he cannot, he gets extremely offensive about it to, he holds on to the question how could God be love if he allows suffering, which is an easy question to answer, if you know the Bible which he does, I know he is making excuses so he can live how he wants. My main question is how can anyone turn away from God like that?, a true believer knows God is real without question, even if I feel like disobeying him I still know he is there, how can my grandad risk his salvation like this?, he is bipolar which might have something to do with his mental state but who knows, my mom says he was never truly saved to begin with and only did it for personal gain, I cannot accept that answer however, because like it said some will turn away in the latter days, and he has given in to every thing it said they would, he is like king Solomon, he knew God but chose to turn his back on him.