"For the Christians, conversion was something that must be voluntary and individual, obtained primarily through preaching and example, and this is how Christianity did in fact spread during its first centuries"
try telling that to all the Jews during the Spanish Inquisition....convert or die....or also try telling that to all the native peoples in North and South America who were forced/coerced to convert. The forced conversion by Islamics against Christians were and are horrible. But please don't ignore all of history.
Luckily the main difference is that in modern times, Christians would never force a conversion today. That's something we can't say about extremist Islam even against other Muslims.
That's an interesting point. You have two major religions that are actively trying to convert as many as possible. They both believe they are the one true faith and as far as I can see neither will stop until there is no one left to convert. I would think a clash is inevitable.
The Spanish Inquisition occurred just after the Spanish evicted the Muslims after 800 years of Muslim domination.