Is Christian faith necessary for Selfless Love?

[Sair]

This is in response to an issue arising in another thread - the claim that Christian faith is necessary for acts of selflessness such as that of Maximilian Kolbe, who willingly gave up his life to Nazi cruelty in Auschwitz so that another man could go on living.

I don't intend this thread to turn into a heated (and potentially abusive) debate over the relative merits of faith or lack of faith; so I would initially try to focus the discussion upon the kinds of motivations that lead us to sacrifice ourselves, in whatever fashion - from the merely inconvenient to the genuinely difficult or even prohibitively hard - for the benefit of others.

In short, is it necessary to believe in Christ in order to act in a truly altruistic manner? If so, what is it, exactly, that Christianity adds to the mix to motivate selflessness? Not least among the relevant considerations are the question of whether love, in any form, can actually be a selfless experience; and is it truly selfless to act in a manner that one believes will be rewarded in an eternal afterlife? Does an act of goodness, charity, or the rendering of significant assistance to others lose its value if one gains benefit from that action, even if the benefit is simply feeling good about oneself?