How can anyone go from being a devout Christian to an atheist?

My sister's husband used to be a very devout Catholic. He used to teach children's catechism classes and went to daily Mass. My sister got pregnant 3 months after getting married to her husband. After their first was born, she got pregnant again 6 months later. Their second child was born just... show more My sister's husband used to be a very devout Catholic. He used to teach children's catechism classes and went to daily Mass.

My sister got pregnant 3 months after getting married to her husband. After their first was born, she got pregnant again 6 months later. Their second child was born just recently.

All of a sudden, my sister's husband turned to atheism a few months after the second child was born. He no longer believes in God. Also, he doesn't want anymore kids. Though, my sister refuses to take artificial contraception because doing so is a mortal sin.

I'm rather confused on how such a devout believer can turn into an atheist. I mean, he was very devout. He prayed the rosary daily and was always talking about the Catholic Faith.

How can anyone go from being a devout Christian to an atheist?
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