As an atheist, when I was on Facebook, it wasn't uncommon for people to unfriend or block me when they saw that I belonged to several atheist groups. A few tried to publicly shame me over it, which never went over well because I think they were hoping to get a rise out of me so that I would block them and then they could act like I couldn't handle a differing opinion. They would get increasingly flustered when I replied calmly and politely to their attacks, and make themselves look foolish and fanatical to even other religious people.

Ultimately, if a person is unable to deal with your differing opinions, then they aren't the sort of people you want to get to know better or associate with beyond basic civility.

There is nothing to "handle". Ignore it like you never even noticed that he blocked you and carry on as normal. You owe no one an apology! If he is hoping to spark a debate/conversation about religion with this act of blocking, don't give him the satisfaction. You have zero obligation to this person, and you have no obligation to coddle anyone's beliefs.