Just do as you've done before, and politely decline. This would you go to the cinema and watch a film you didn't want to see, you say yes, but you could add, my wife would know better than to ask me, or I'd spend my time making lists in my head for work/hobby and wouldn't be listening to the service. Tell him your happy that he's found something that helps him, but it isn't for everyone, and you happen to be one of those people. If he starts on about rotting in hell etc, just smile and say you'll take your chances, afterall, nobody has come back with any definate answers, have they. If he keeps on after that, then tell him you wish him well, but you think the friendhip has run it's course, and nothing is going to make you chane your mind on religion. All the best!!