Or is it the Christians who share the message with you?
This is what I find offensive:
Smug Christians who believe that, if only I hear the story of the gospel one more time, I will suddenly and miraculously believe, unlike the previous thousand times I've heard it.
Christians who act as if they are some oppressed sub-culture, when in fact that are the dominant religion in my country and in much of the world, and all too often willing to oppress those who believe differently from them.
Obnoxious Christians who want to "witness" to me for their own satisfaction, despite the fact that they've been told before that I have heard the message already and that I reject it.
Dishonest Christians who insist on trotting out the same discredited arguments for their faith time and again, no matter how many time those arguments have been dispensed with.
Self-satisfied Christians who act as if my non-belief were the product of some failing on my part to understand the story of the gospel rather than an informed conclusion which I have every right to come to myself.
But most of all, I find offensive those dense, self-centered, blinkered evangelical Christians who just will not stop preaching to anyone and everyone within range, no matter how many times they have been told that such behavior is not welcome.