Why do Christians need to promote their religion to non-believers? Door to door Jehovah's Witnesses / Mormons, people preaching in town centres, posters outside churches, alpha course.

Why do  Christians have to promote their beliefs to other people? What difference does it make to believers if other people do not believe the same as them? Do they not think they may offend atheists?

I am interested to hear from people who actively promote their religion to non-believers and what drives them.
You basically have three sub-questions here, so let me attempt to answer them in order.


  Why do christians have to promote their beliefs to other people?


For two principal reasons: because we were commanded to at several different places in the Bible, and as an act of love.  The Gospel improves the lives of those who live its principles, and we want to share that with those who don't have those blessings in their lives.


  What difference does it make to believers if other people do not believe the same as them?


Well, aside from what I posted above about loving others and wanting them to experience the same blessings we have, it makes a huge difference when the other people are not content to simply "not believe the same" and leave it at that, but actively attempt to use force--of either the physical, political or legal variety--to interfere with our ability to live our religion.  And one of the simplest ways to counter that is to turn an enemy into a believer.  (Sometimes it can have a very dramatic result; just look at Paul!)


  Do they not think they may offend atheists?


Perhaps, but that only matters if you think that avoiding hurting someone's feelings is more important than sincerely trying to help them improve their lives.  The Gospel encourages an eternal perspective, which tends to give us a different ordering of priorities than the strong short-term focus so prevalent in the world today.