But a person can act in a truly altruistic manner even if, for example, they are not a professed Christian.
He who knows Love, knows God ----Even if he worships God "as unknown".
Let me add my response with two bible references:
IMHO, it just that Christianity adds a level of knowledge, understanding and clarity that may not be available to the person who chooses to act lovingly but is not sure why or how. The teachings of the Church can help a person sort through the sometimes difficult and contradictory impulses and inputs and feelings that we have to deal with in discerning what actually IS the most Loving thing to do in a given situation.
The best answer I can say on this is that "Love" and "expectation of reward", while not mutually exclusive, tend to be at opposite ends of the Spiritual journey. Often times one begins the journey in order to avoid punishment. This is more "fear" based, but then we are told in Scripture that "Fear of the Lord is the beginning of wisdom".
As one moves along the journey, seeking to immerse oneself in Christ, to emulate Him and make Him Lord and center of our lives, this fear of punishment is replaced by a deeper and more truly formed Love. Love then drives out fear.
Spiritual writers often refer to the relationship in nuptial terms. Jesus is the Bridegroom we are the bride. A husband and wife, truly and fully in Love, need not have expectation of reward from the other. This is because each one has, as their first priority, the best interests of the other. Their actions toward the other are not in the form of "labor for wages", or out of fear of punishment, but rather they are gifts freely given out of a desire for the other's highest good. The reciprocity of such a relationship might look like each being "fair" to the other, but in truth neither thinks of "fair" but only of the good of the other.
Of course the above doesn't even begin to scratch the surface of the relationship between God and man...Or how and why saintly people do things out of Love without thought to recompense....But it's the best I can say for now....
But if I'm a guy, does that mean I gay-married Jesus?
I'm obviously kidding, but that analogy just goes to show how silly gender roles and the condemnation of homosexuals is.
So let me see if I have this right: even if a person doesn't believe in a god, when they do good, it's because of Jesus?