The ground of Christian and Jewish morality is found in the 10 Commandments, which describe what it is to love God and love your fellow man. Athiests skip the first half of the commandments—then they tend to follow (more or less) the 2nd half of the commandments. In denying the ground of love of man, which is love of God, it is no wonder that they erode even love of man....justifying murder (in abortion, euthanasia, & etc.) sexual immorality, theft (see Wallstreet), lying, coveteousness, etc.
Without love of God, you cannot have a solid, lasting love for your fellow humans.
The natural law, which is pre-political, is imprinted on the souls of every human being by God. An atheist can live a moral life by recognizing and living by the natural law, even if they don’t know God. IAW, they can live a moral life because of the gift God has given them.