He wondered if I was the right person for him
It is a blessing this has come up now rather than later.
has a huge problem with the church and would not accomodate a Catholic lifestyle
This is, basically, a deal breaker. Again a blessing it has come up now.
I love him. I can’t leave him.
Of course you can. You aren’t married. He’s told you what he thinks of religion— of your deeply held beliefs. This isn’t a person with whom you will be happy in the long term.
This is what dating is for— discerning whether you are made for marriage to each other. It is not easy, but it is better that these things are discovered before marriage. In such cases, dating has done it’s job, weeding out those who are not compatible.
now he is watching atheist videos on Youtube so I can hear them.
Well that’s super mature and respectful. <in case it isn’t clear, that was sarcasm>.
Yet another indicator he’s (a) not ready for marriage and (b) not a good choice for someone of any faith.
You are going to have to get some perspective to see that yes you can leave him. If you are living together you should stop sexual intimacy and move physically if you can— not necessarily to end the relationship entirely but to put it on better footing so you can see clearly. Although given what you’ve described, I don’t think this is the stuff of long term success.
What feels terrible today, will feel like a bullet dodged with time and perspective.