I see it like telemarketing; it's your job to sell me something, and I don't begrudge you having a job to do. However, after I politely decline interest in what you are selling, it's your job to stop selling me. Continuing to sell me after I've politely declined give me a right to shut you down completely, whether that's hanging up the phone or walking away from a conversation.
With that being said, I am fascinated by the religious beliefs of others, and usually enjoy hearing about them, as long as the conversation is even-handed and respectful. If your only goal is to convince me that you're right and I'm wrong, our conversation is going to be very short.