Say two people get married. Woman is from non-religious background and man is raised hard core Muslim.
They both don't care about religion. Woman believes in some form of God but she is not part of any particular religion. Man does not want to be a Muslim and that was never an issue between them.
Now two things happen!
First woman gets pregnant to have a child against all odds. Starts to believe even more in blessing.
Man stumbles to a group of men who claim to be ex-Muslims, agnostics and humanists. He believes even less in existence of God now.
They both turn into extreme. Believer and non-believer.
Do you think that marriage can work?
That particular didn't and try to guess why!
You have all colors of the spectrum in any religion or lack of religion. From extreme will degrade you for your beliefs to can look past a persons religion for who they are. I personally hate the extreme weather he/she is an extreme Catholic, Jewish person, or Atheist. I believe two extreme people cannot work. I picture them both pushing what they believe onto each other and whenever a fight starts it turns into a religious fight and becomes verbally abusive and degrading on both sides. That's what I bet happened in that instance and someone just walked out on someone.