Abiathar, neither theism or atheism are needed for human beings to be "motivated to do good."
The only motivation needed to do good is listen to our natural, in-built instincts. To listen to our conscience. We all have a sense of right and wrong and follow basic ethics. Most humans by default have no desire to harm other people, realising that such harm can easily come back upon themselves. We understand that we harm ourselves when we harm others, and we are all born with conscience, a sense of compassion and empathy for the suffering of others, regret and remorse for when we cause suffering to others. When we witness terrible suffering of others we immediately empathise, because we are all the same and nobody wants to suffer. Family, community and society cannot function when we go around stealing, harming and killing each other.
This is the simple foundation of all ethics and morality: empathy, compassion.
No theories or science is needed.