As far as I know from a Christians point of view, conversion is considered a miracle and thus only God can truly convert a person (and because God respects our free will, he does it only if the person wants to be converted).
On the other hand, helping others to reach heaven is considered a good deed, so even if it's not obligatory, a Christian should try to help others establish/reestablish their connection with God.
And from what I've been taught, the best you can do to achieve that is try to love everyone (even satanists!), pray for them and give example with your own life. A logical conclusion is that you should 'preach' only if the person wants it, or in rare situations when he does not, but you think it's really for his good (similar to a situation when you do something against a friends will, or a parent does something against his child will to save them from harm).
Also, as far as I know, crusades and inquisition (as well as similar situations in other religions like Al Kaida) were in fact lead by nonbelievers as they are so contradictory to the teachings of their religion, that anyone who does indeed love God would clearly see the problem. They simply used religion as a mean of manipulation to achieve their goals. There were even cases in Middle Ages when a Pope was a non believer who achieved his rank through noble birth, wealth and connections, rather than faith.
Nowadays I guess most of the priests are true believers, although still there are exceptions, but at least it's much less common among bishops and higher ranks.
Heh... If I were to say which event was the worst in Christianity's history, I'd say it was the "Edict of Thessalonica" that made Christianity the state religion of Roman Empire and thus obligatory. Not only it was wrong as it forced people to convert (which obviously does not work and people would join Christianity only superficially, but keep to their believes secretly), but also it was extremely harmful to the Church itself as it was the moment when non believers started to call themselves Christians and thus their actions were considered representative of Christians even though they were not.
And just to make it clear, I'm not saying that Christians are any better than non believers. I simply think that a person who does believe in 'God who is love' can see that killing non believers (or persecuting them in any way) is bad, while someone who does not believe in God might not necessarily see it so (although he can). Of course, whether the Christian does not kill because he knows it's bad, or whether he does commit to sin, is another matter.
