I’m concerned 😰

My fiancé today told me that he is not Christian. He called Christians stupid. He got really angry at Sunday school teacher and Sunday school classmates. He is right now ranting to his mentee a young guy 18 years old (fiancé is almost 27) he’s ranting to the guy who is an atheist about church saying it’s not his scene and he understands why he his mentee won’t go to church because everyone is ignorant and stupid and against him. He wasn’t always this way. If anything he way a Bible banging preacher boy in high school and the begging him or college. It’s been a slow spiral but in roller coaster form so I guess that’s why I didn’t realize how serious this was until now. My fiancé does speak out of emotion 80% of the time but he has not come down off this all day and hearing him agree with the atheist is concerning. I’m shocked. I want to support him but he is not in the position to receive from me and hasn’t been for a while😕. He only wants me to agree with him but as a Christian I honestly cannot. What do I do? I know I can pray over him and I will. But I’m shook. He’s never gone THIS far. To call up his atheist friend to agree with him rather than encourage him. He’s been going on and on talking to whoever will listen about what other people said but twisting the words completely to fit his narrative. Careful not to tell the same story to anyone who was actually there and heard the same words as him. What’s going on? Who is this? Please pray for us. Thank you.