[OMFG!You'reActuallySmart!]

I would tell my future child that I wouldn't mind whatever he/she believes in as long as it makes him/her happy, understand it and don't push it on everyone they see. Also, I'd always remind them that no religion has any actual proof in my view and regardless of their religious choice, I'd love them nonetheless.

Since your kid is 18 months, he cannot understand nor appreciate what any religion truly is. Instead, what I'd do for my future child would be put them into a religious class(es) at school. I would always encourage them to ask questions and if he/she asked me a question about religion, I would answer it as best as I could while "dumbing down" the content so they could understand it.

As for religious relatives, I'd tell them that I'm going to raise my child in a healthy way as I see fit and would appreciate they don't attempt to indoctrinate him/her. If they do attempt to, I'd remind them that although I do want my child to learn of religions, I don't want him/her to know only of one religion and no others. I'd let them educate but not indoctrinate him/her. If they refuse and keep indoctrinating him/her, I'd tell them to go fist themselves and if they want the child to develop in a healthy relationship, they quit what they're doing, otherwise I'll go as far as allowing them to see him/her only when I'm present in the immediate area.