It seems pretty obvious that morality is a learned pattern. Kids naturally are instilled with the popular morality of the time - and it’s ingrained on them so frequently and generally consistently that it becomes second nature.
“Mommy, why can’t I take that?” “It’s not yours” “But…” “that would be stealing. Stealing is wrong”.
Back when we were first hauling our asses out of caves, rape would probably be perfectly societally acceptable. Today?.. ect.
Setting aside for the moment the question of whether it proves the existence of God - can anyone bring up a society with a really different idea of morality? Where cowardice, greed, cruelty, vanity or dishonesty are considered virtues? Where it’s considered admirable to tyrannize widows and orphans, show disrespect to your family and betray people who are kind to you? I know there are plenty of societies where it’s acceptable to treat certain groups badly, but I don’t consider that to be a truly different concept of morality, just a different idea about what entities warrant moral behavior.
And, yes, this idea as an argument for the existence of God is explained better than any of us could do it in Mere Christianity - but I believe there’s another C.S. Lewis work in which he develops this argument even further. The Abolition of Man, maybe? Any other Lewis fans know?