The biggest indicator in your sitch is his saying there is no God.
In my opinion, 97.5% of atheists say they believe there is no God because then they can justify anything they want to do.
Your H wants to leave, get a D, potentially date and sleep with other people. That is contrary to his spiritual beliefs. So in order to make himself feel like what he is doing isn't wrong, he changes his spiritual beliefs to fit what he wants to do.
I see this all the time. Biblical teachings are sometimes ignored because someone, or a group, or even entire denominations do not like what that teaching says. (The roles of men and women in the Church and in the home are perfect examples of that.)
So his saying there is no God is a symptom, not a cause. And talking to him about what his spiritual beliefs used to be will not sway him. He is running on feelings now, not logical or reason.
If you can stay the course, give him space, concentrate on what you can control, YOU, maybe there is a way through this. You said it happened before and he eventually returned to the MR. So you know that it can happen again (him returning to the MR).
Keep posting. I will take a more active interest in your thread. Sorry you felt ignored, the people here are good folks and I don't think anyone was ignoring you on purpose.
Finally, realize there is no magic solution to your problem. The key is to prepare you for any and all eventualities. Whether he stays or goes, you need to do the work that will make sure you are taken care of.
M(53), W(54),D(19) M-23, T-25 Bomb Drop - Dec.23, 2017 Ring and Piecing since March 2018