Hello, and welcome to the forums!
First, I am so very sorry that you are in such pain. Secret affairs are never OK. They cause so much harm and pain. I can relate, as my ex husband, who is also a narcissist, cheated on me.
Secondly, and I emphasize this point - do not try to impress your religious beliefs or faith upon him or her and do not try to "help" them see the error of their ways. If they had any morals or sense of human decency, they would never have done this in the first place.
And lastly, I know it won't seem like this now, but this ending and affair is to me a blessing in disguise. Narcissists do not change and only cause great harm. You are now free and will be free - to live a far happier life without that toxicity around you.
Commit to a healing path, and live your life. Go out, do things, see friends, exercise, and do plenty of self-care activities. Show yourself compassion and self love.
I had to go through a divorce and have been divorced for over two years. I am still on the healing path, and am carving out a life of my own.
You WILL and CAN get through this, as hellish as it seems now. Hugs to you.