Every man must act in a way that produces the consequences he desires.
There is no need for god in that question other than to give one excuses for having taken actions that others disagree with.
What you mean is that a man must provide his own moral authority.
In other words, one needs justification only because one is either weak, or because one is demonstrably wrong because it causes retaliation from others.
If one is strong and one is right in that he does not cause retaliation from others, then he needs no external authority.
So in general, what we see is that those who do not obtain status from others, or do not obtain the status that they think they warrant, seek to obtain self-image through creating niche narratives in which they envision themselves heroic or of high status.
Since many of us need these lies, because the admission of our status as much lower than we envision, and our abilities much lower than we envision, we must morally tolerate these comforting lies in the private sphere just as we tolerate the comforting lies of religion in the private sphere.
The question arises as to whether we can tolerate these nonsense ideas in the public sphere.
And this is where we get into the problem. Wherein the lies people like you ritualize, using pseudo-scientific pseudo-secular language, can be so real to you – through the social construction of reality – that you can apply these PERSONAL needs to arguments in the political sphere.
SO whenever your comforting lies produce harmful externalities, then it becomes a matter of dispute resolution between different sets of comforting lies.
And to resolve a dispute between sets of comforting lies, we need a means of decidability. That means of decidability is what we call ‘truth’. Meaning in the social context: are your statements free of error, bias, wishful thinking, suggestion, and deception, and are you trying to use those words to impose costs on others or escape costs you yourself should bear.
Because truth and objective morality are identical propositions.
Anyway. This is probably too much for you; but you put in honest effort and you haven’t (knowingly) engaged in trickery during this discussion, so I have to take you as an honest man that is merely trying to fight above his weight class.
I don’t really care because an honest man, even one who believes silly things, is better than a dishonest man. And that is enough.