Perhaps the religious sense and the moral sense both stem from the same impulse, an idea on which Freud has written a few interesting words. Those that instill fear of immediate or eternal punishment and hope of reward as a basis of moral behavior hardly provide an encouraging alternate vision to those who suggest a moral sense stemming from intelligence, cooperation, reflection, wisdom.
If that is your description of Christianity and secular humanism, respectively, then I reject both characterizations.
The question still is: What motivates those to suggest either? If you believe the logic of E.O. Wilson, it's not what they think. Once again, from the article:
“Evolution in a pure Darwinian world has no goal or purpose: the exclusive driving force is random mutations sorted out by natural selection from one generation to the next. … However elevated in power over the rest of life, however exalted in self-image, we were descended from animals by the same blind force that created those animals. …”5
and
Ethics as we understand it is an illusion fobbed on us by our genes to get us to cooperate.”6