Again, if God, like Jupiter in the comedy, should, on awaking from a lengthened slumber, desire to rescue the human race from evil, why did He send this Spirit of which you speak into one corner (of the earth)? He ought to have breathed it alike into many bodies, and have sent them out into all the world.
The above quotation is from one of Christianity’s earliest and most stringent critics, the 2nd century philosopher named Celsus. His irritation with Christianity is more than evident here: how could such a backwater sect make claims to revelation of universal importance? Such claims were only more audacious in Celsus’ eyes because he considered Christianity to be composed entirely of anti-intellectual simpletons who preyed on the weak. I’m not sure some of Christianity’s contemporary critics have moved away this position.
In fact, the notion that one group of people can claim to be the sole possessors of an important universal truth has remained odious in the eyes of those on the outside of that group and, if anything, has only increased as our society becomes more diverse and so many religious and cultural traditions live, quite literally, side by side. How arrogant, some exclaim, does someone need to be to claim that they have the absolute truth about God and reality. Christian missionaries are regularly portrayed in a negative light, with critics pointing out the fact that cultural triumphalism and imperial motivations often accompanied European missionaries in the early modern period. Indeed, the many stories of disastrous “missions” to South and Central America that involved the Conquistadors are well known and terribly sad. One might also mention heinous acts like the forced “Christianization” (which really just meant “Westernization”) of Aboriginals in Australia or Native Americans in North America. All these things are true, and they are truly heartbreaking.
However, even when mistakes and atrocities are admitted and we Christians backpedal as fast as possible from arrogant presentations of our religious views, it remains the case that Christianity is, at its root, a missionary religion. In Matthew 28:19-20, Jesus says to his disciples “Go and make disciples of all nations.” The earliest Christian writings that we have come from a pioneering missionary and pastor, Paul the Apostle. Within about 300 years or so of its founding, Christianity spread from provincial Judaea to become the national religion of Rome, the world power at the time, thanks to a controversial move by Emperor Constantine. It soon spread from Rome to the various tribal groups in Western Europe and, by the time Islam arose in the 7th century, Christianity stretched from Ireland, down into Africa, to the Arabian Peninsula and beyond.
In more recent times, some Christian groups have tried to formulate a more nuanced and charitable position towards other religious views. For example, some propose the idea that people might be “Christians” without really knowing it. Such efforts at charitable inter-religious dialogue are a relatively recent phenomenon, though I certainly think they are good. However, as offensive as it might seem to those who are not Christians, it remains the case that Christianity proclaims that there was a certain Jewish man named Jesus who was more than he seemed: he was the one God himself who condescended to become human with us in order to save us from ourselves, death, and destruction. It declares that God is the only living and true God  and that being “in Christ” is the only way to be reconciled with God in spite of our sinful nature.
Be that as it may, to anyone reading this who are not Christians, please know that even if you find it absurd that some person you meet tries to share their Christian beliefs with you, it is done because they care about you. As comedian Penn Jillette, an avowed atheist, asked, “how much do you have hate somebody to believe that everlasting life is possible, and not tell them that.” If anything, take your friend’s proselytizing efforts as a sign of affection, they love you enough to share the source of all love with you.
 Quoted by Origen in Contra Celsum 6.78. A handy set of excerpts containing Celsus’ comments can be found here.
 See Origen’s Contra Celsum 3.44, 55, 59, etc.
 This has cropped up, for example, is such different places as the (Catholic) Vatican II council and in (Protestant) C.S. Lewis’ Narnia series.
 See for example 1 Thessalonians 1:9-10.
 See John 15:1-5. Of course, this blog post is a highly compressed account and precisely what it means to be “in Christ” is difficult to define.
 I admit that sometimes this is not the case and that people present Christianity in order to bolster their own pride or feel good about themselves. I don’t condone this but I prefer to give people the benefit of the doubt, hence my positive account here.