In the case of Jimmy Carter, I don't think you can separate the work he does for human rights and his concept of himself as a Christian. I think those two things are probably inextricably linked in his way of understanding himself and his motivations.

I can say the same for myself. I'm not a Christian, but my understanding of my spirituality is a prime motivator for the way I live my life, and the things I feel strongly about and advocate for (or against).

This is not to diminish the contribution made by the freethinking secularists. Their motivators are no less valid than those of people who have spiritually based motivators.