It’s sort of a quagmire, but if you are speaking about “progressive” atheists and humanists, the morality that is espoused, simply put, seems to be based on empowerment and tolerance. Everyone is free to do what they want and feel good about themselves and the greatest “sin” is to interfere with another person’s doing so. Where it gets complicated has to do with boundaries, where one person’s freedom interferes with that of the other. The ultimate good is in being oneself. That is where again things get hazy because the will needs a master, a direction, a purpose or end to achieve. While it is up to the individual to arrive at that meaning for their existence, it must exist outside, transcend the person in order for it to have any sort of influence on one’s will. Survival of and harmony within the human species would be one foundation of atheistic morality. Noble aspirations to be sure, but they would not hold much weight in situations where one must make a difficult moral choice, especially where it is known only to oneself. This sort of morality seems more useful in judging others, such as politicians and others in the class that holds power and influence. It’s also a way to feel morally superior, as an atheist might feel intellectually superior to the believer. Bottom line: I don’t know either.
My default understanding of a phrase like “common sense morality” is that it would have to do with conscience. Conscience here is understood to be our dialogue with God, which we all have since He brings us all into being through His love. When it is not recognized as such, it is less likely to be informed. That said, we all are likely to feel compelled to become more conscientious when we enter into conflict within ourselves and our difficult dealing with others. Another alternative, of course, is to deafen ourselves to its message. The problem with morality has to do with the ignorance that plagues the human condition. The voice of our conscience melds and is confused with immediate and tangible realities such as the people with whom we interact as part of a family within the greater society. Our conscience, left unexplored can become simply a collection of “gut reactions”, the emotional sequelae of interactions with significant people in our lives. Without a connection to what is truly good, from which follows the knowledge of sin within ourselves, we will drift either into sociopathy or will be left with what are merely irrational feelings of shame and guilt, these accompanied by a flimsy sense of self-esteem that replaces the reality of our being loved by He who brought all into existence and wants us to join Him in love.
As to nature and those who hold us to be animals, there is the herd. In addition to instincts directed to self-survival, I would postulate that feelings arose (“were created” would be a better way to phrase it since they did not bring themselves into existence) in the “evolution” of the animal soul/mind. At some point instincts dedicated to the care of the young were strong enough to supercede those of self-preservation. The inclination to this sort of behaviour would have then be generalized to others forming the group.This isn’t strictly altruism, which involves a choice, since it is simply a different emotional reaction. The same feelings that lead us to work together in harmony can cause a holocaust. This occurs through the surrendering of one’s will to society, which swallows up one’s identity and freedom. Actually, it is only within the Church, the body of Christ, here as a sample of heaven, that this does not happen. Human beings are able to choose how we behave, we go beyond the vagaries and conflicts of feelings, adhering to standards that see one action as being better than another. Although we possess a similar anatomy, having lungs, a heart, brain, muscles and bones, and we likewise share some of their feelings, our eternal human souls, with its capacity to know and relate to God, makes us something other than animals.
At the foundation of Christian morality, we have Love, God, the Source and destiny, should we choose it, of our lives. The Ten Commandments are founded on the two that express this truth. They are practical descriptions of how we, divorced from the truth of love, can make it central to our lives, as it should have been. Behaving in this manner, we elevate this earth, our relationships, to heaven, where God’s will is done. Entering into His infinite compassion, we see ever clearer things and people as they truly are. Unfettered from the illusions that accompany sin, the world in all it’s wonder is revealed and we can honestly love our enemies.