Following or not following any religion is choice everyone should be allowed to make for him or herself. It's his choice to make. It might be permanent or temporary. Be there for him when he needs you regardless of whatever decision he makes on this subject. (just my 2 cents)

Both my sons were raised to be critical thinkers, to question, to research using reputable sites & citations & to be able to lay out a well founded arguement based on fact. They were both exposed to religion & going to church. They both became agnostics in their teens & only more skeptical as they have grown older (both now in their 30s).

If they started to gravitate to religion, those would be interesting conversations.

How old? Old enough to make decisions? Mother involved? Lot of variables.

Who is she?

Swat them with a rolled up newspaper, and tell them, "NO! Bad teenager! BAD!!"?

Point him to people whose lives have been ruined by it?‍. Remind him he only gets one life, and encourage him not to waste it on insanity...?

I would ask why? And move on.

I couldn't answer that as I have no children, but if I did and they started believing in imaginary people, a nice long chat would be in due course.

“Hold on a goddamn minute.” ?

I'd tell him it's completely unacceptable and you raised him to be smarter than that.

I'd take something like the Socratic approach. Ask him questions concerning the things make evangelism compelling. After a rich and honest discussion, one of us will be convinced.

What you resist persists

Good luck with that. Do what your heart says to do.

Stone him to death--it's Biblical.

You say, "What dat chit uon, man?"

I would say, "Boy, what the hell is wrong with you?"

I would sit him down and ask him many probing questions about what he understands about the ideology he is buy in to. The choice is his. Your job is make sure that he is making an informed choice.

'Holy Shit!'
Sadly, I'm not kidding.
When my son was in pre-school the kids next door told him Jesus was in the clouds, looking down at them every day...serious bullshit. I took him outside and we looked at the clouds and we didn't see anyone up there.
Hope you go at this hard (NOT hard on him but don't be wishy-washy, tell him what you want for him, what you believe and why this concerns you assuming it does). Regularly engage him: find out why it's appealling, who's the influence? What is happening in his life that he's connecting in that way? I really think that if a kid has someone to help them through, they don't buy into bullshit and evangelism IS bullshit. Evangelism is cultish. The parent has to be dilligent, it may not clear up with one discussion and he might go this route anyway. Hope you give us some followup, Nevadarich.

“Get Out!”
Just kidding of course.

I have that fear every day. I have two step sons that have been with me since they were babies, and their bio father is religious. So alternate views are inevitable in my house.
I have always told them, I will love you no matter what, but don’t expect me not to challenge you with whatever you believe. Even if it’s the same as I believe.
I tell them it’s more important to know HOW to think, rather than WHAT to think.
But that’s tough to watch. I know some don’t feel as strong as I do about it, but to me it’s almost the equivalent of watching a loved one struggle with drug addiction.
Good luck to you sir.

I would tell him that there are these "special summer camps" that will fix him..

Tell him to read/study that book , after that he can make up his mind

Everynbody learns in their own way. However, is there any chance he might be becoming schizophrenic, or have other problems? Tread carefully, but remain aware!

Read more books and study Roman history

If you follow any religion, on a global scale more people will think that you are bat shit crazy than not. It's not a great argument though as if you were to place the word don't before follow, it's still true.