To address this question of compatibility between what one would call morality and a non-belief in God or any form of supernatural higher power theist underlies the existence of the universe/s the first question that really has to be dealt with is what do we mean by “morality” and how it would be relevant to either context.
Moral Law or the Karmic Law
In the atheist’s view of morality, humanity gains it’s moral awareness from the non-physical presence of God or the higher power which is referred to as the Moral Law or the Karmic Law and this will provide moral yardstick for which all intentions, actions, and deeds are based upon. Humans under this system unlike animals are in a privileged position of where they can discern this underlying foundation of existence and through this knowledge gain a degree of independent freewill to decide between good and moral or evil and immoral acts.
Within this scenario the moral or karmic law would still exist and apply in some way even if say asteroid impact was to wipe out all life on Earth as the foundational moral principal has it’s within God himself and the human mind merely knows and understands the will and character of God. This way morality would be attained through thoughts and deeds that align to this eternal non-visible power within and/or beyond the universe itself and immorality or sin would be thoughts and deeds that would be opposed to the Law of God or the Karmic Law.
This way morality is defined as an objective reality that in some way exists independently in it’s own right and it would therefore not be purely a human-made concept or invention. Human made concepts and inventions regarding morality would be formed around the principal central Moral Law which would be defined by God which will then be instilled within a certain number of his creations, beings such as humans who have a certain degree of capacity for rational rather than purely instinctual thought.
When we turn to the philosophy of atheism which rejects outright the existence of a God or non-physical higher power of any kind or at least posits that there wouldn’t be a good reason to actively believe in the existence of such an entity or force due to a lack of compelling arguments or physical evidence the view of morality becomes something altogether different.
How is Morality Defined in the Atheist System?
Under the atheist system morality has no form of existence beyond the confines of individual human brains and would therefore become something entirely subjective to each individual to decide upon their own moral standards as they choose or see fit. However, as social animals, humans have to live in societies and depend upon each other to survive. In this situation, individual survival prospects are enhanced by following a principal of least harm morality being purely defined amount of suffering or assistance given to other humans or living beings on Earth as there can be no other way to define the meaning of what it would be to be moral or live a moral life.
And indeed we see other social animals in the wild adopt this very same survival strategy bearing in mind that unlike humans animals can’t appreciate the concept of morality intellectually it’s some they have merely evolved to do over millions of generations for mutual protection and survival advantage. As a rule “Do onto others as you would have done to yourself” would be the golden rule for co-operative survival the natural world. In certain situations it is of course to your an animals survival advantage to inflict suffering on others, if a lion attacks kills and eats you it may seem like it’s not being particularly nice but by no means is the lion doing anything wrong or immoral it’s just following its predatory and/or defensive instincts. Also when a male lion defeats a rival and claims his breeding females he will ensure to kill all the male lions young previously born to his females to ensure they are brought back into mating readiness.
This again may seem like grossly immoral or evil behavior to us, but for a lion it’s just what they have evolved as part of their instinct to survive and pass on their genetic material. When it comes to human morality of acts of compassion and kindness and act of cruelty and evil humans would though more intelligent than the lion or other animals that follow social co-operative social rules would really just be carrying out the same instincts for survival.
It is therefore highly debatable if morality, as it has been traditionally seen within a theistic or spiritual context, would genuinely still apply to anyone in a universe of natural laws and physical matter and the byproduct of natural laws and matter as in the conscious existence of a living being for example. In either scenario an atheist would be entirely equally as capable of having the same sense of moral values and right and wrong as the society in which they live and/or were born into whether moral value is ultimately derived from God or from natural evolution.
But only in the first scenario would an atheist or anyone else for that matter have a genuine capacity for moral choice which isn’t purely based on bio-chemical and neurological determined processes or subject to individual or cultural opinions based around evolutionary hard-wiring of the physical human brain. The human brain being the fundamental source of an individual’s conscious self in the naturalist philosophical sense, this being programmable in the same way as a computer by their upbringing and wider society.