Though the altruistic approach seems good on the surface, even to theists, especially Christians, upon further reflection on how this value actually plays out in the real world, one can see the cracks in the secular altruist's argument. Mainly, how do we define what is good for all of society? We as individuals, cannot determine what is best for society, for we can never have a complete enough picture of what society most needs, at least not to a moral certainty. Moreover, is the simple fact that no individual can act on behalf of society.
Altruism seems good, even Biblical, but cannot serve as a code of ethics. The problem with altruism isn't that self-sacrifice is bad, it's that it allows society, not the individual, to make the call on who is required to sacrifice their own best interests and why. Society is the majority, and altruism makes one's actions subject to the need of the collective.