If he went to school to be a burglar would you congratulate him? His new goal is to steal the minds of children. To tell fairytales that will be supported by the mindless flock. I would not be confrontational, but I’d never congratulate him.

I'd remain quiet.

That's tough. On one hand I understand what you're going through. But in hindsight of how you feel, I myself, religion does serve a good purpose to a degree. There are people that religion help in a lot of cases, communities also. Some people have over come great odds, trials, and misfortune, with the help of religion. Outside of the millionaire billionaire giants of religion especially. I'd have to go with a congratulations in the mind set of giving him what he'll be giving. A little hypocrisy with a mention of, do as much good as you can with it.

Congratulate him, of course. No need to launch into any discussion about anything at all.

honestly you just give him the congratulations and you leave it at that. He's your family so you treat him nicely but you don't have to like the fact that you treated nicely. Just cuz he chooses to screw up his life doesn't mean you have to let him screw up yours

I would say,, "if this is what you want, I wish you joy and satisfaction. Please remember the Hippocratic oath for doctors: "Do no harm!"

I think if you're happy for him and his accomplishment, go ahead and tell him so. You can support him without necessarily supporting what he does. (Think of it like a parent finding out their child is a successful porn star; sure, it might not be a career they'd have wished for their child, but they can be supportive of the achievement.)

Respect his decision, treat him like family and speak your mind as far as you can.

Personally, I would congratulate him on achieving his goal. I go to my grandchildrens' baptisms even though I hate the fact their parents are raising them religious. I just look at it as being kind.