As Francis says, it's part of the remit of many religions to 'spread the word'. You say religion is a personal thing, but if you really think about it, it isn't. It affects everyone - whether they want it to or not.
I had a couple of Jehovah's Witnesses on my doorstep a few weeks ago, and was in the mood to challenge them. Fortunately they were carrying a bible, and so without bringing mine to the door, I was able to show them much that they were unaware of. They went away still clinging to their fanciful beliefs and in complete denial of the evidence - as usual. I hope I gave them something to think about though. After all, if they feel at liberty to 'spread the word', I can't see why the rest of us shouldn't.