What is a nice polite , yet successful way to ask for someones email address?

I'd say that's an unrealistic quota, offering some incentive for giving their email address is important, there should be something you will send email to them, that they will want.

I'm not sure if asking for an email meets the Can-Spam regulation requirements for someone to opt-in to an email list before you an send commercial solicitations to them, there may be a certain line of legaleze you may have to quote, on include in the followup email.

The best thing to do is not even try logic or respond at all. That's why most religious people turn others away from religion all together. They present themselves with no grace or dignity and are hostile and argumentative. Don't even open further messages from this person so you can avoid the ire and temptation to go off on them. Show you have more grace by being non-responsive. After all, that's what a REAL and TRUE Christian or follower of the techings of Jesus would do. Even if you aren't religious, show them that example so maybe they'll take a hint.

May I have your email? I promise you will not get spam from this company or any third-party clients.

May I have your email to keep you informed about our company or other offers?

May I have your email? We do not sell your email to other clients or businesses.