I assume that that statement applies to the conditions faced by the immediate readers of Matthew, and the restriction was retroactively put in Jesus's mouth to give the warning authority for that community.

Of course there are plenty of differences between Christian groups, but I'm talking mainly about Christianity itself, not Christians. I'm saying the essence of this religion itself is annoying. We can argue about what's essential to Christianity, but I'd just say that any sect that takes the gospels to be historical rather than mythical, and that goes out of its way to convert non-Christians is bound to be annoying for the reasons I give here.