My sole rational argument for Christianity is that it has made the societies that it is, or was, infused with, more honest than the ones that were not.
I also don’t believe Christian societies are more honest than others. Every religion I know of teaches honesty, as does every non-religious ethical framework I can think of.
In Christian theory, everyone is a sinner, but in a real Christian society, people try to cover up their particular sins all the time, at least against other laypeople (not the priest), which leads to the opposite of honesty - hypocrisy.
also the crusades were in significant part a competitive reaction to Islamic "proselytization via violence" so you're placing the blame on the wrong religion, there