Earlier quoted context omitted.
God is not going to smite you for oath-breaking either though. What matters is that you keep your promise to (in this case) society, not whether you swear it on any book in particular. If the law requires the promise to be sworn over a book because the lawmakers at the time were religionists, you might as well choose a book you think represents your values.
The point is there is a belief in a consequence for lying to God. Whether or not there is an actual consequence is irrelevant. Furthermore, you can’t prove what happens after you die. I mean you may very well be right, but it’s just your personal belief.
That is a belief in personal responsibility not motivated by fear of punishment by a supreme being.
This quote from Sagan convey's that meaning among others IMO:
It has been said that astronomy is a humbling and character-building experience. There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly with one another, and to preserve and cherish the pale blue dot, the only home we've ever known.
Also see
https://en.wikipedia.org/wiki/Lawrence_Kohlberg%27s_stages_o...
Where fear of gods punishement would be at the lowest level.