The man who has sought to master his flesh according to the counsel of God should know better than to believe atheists and agnostics who claim that men will master themselves out of no more than a sense of fairness and ethics.
The one way we can be most certain such men will not master themselves is in matters of chastity. And if they will not master themselves in that, they will end up under the power of the adversary.
Mosiah 16: 2 And then shall the wicked be cast out, and they shall have cause to howl, and weep, and wail, and gnash their teeth; and this because they would not hearken unto the voice of the Lord; therefore the Lord redeemeth them not.
3 For they are carnal and devilish, and the devil has power over them; yea, even that old serpent that did beguile our first parents, which was the cause of their fall; which was the cause of all mankind becoming carnal, sensual, devilish, knowing evil from good, subjecting themselves to the devil.
Here we read of the misery of the wicked. Of course they cannot dwell in heaven while the devil has power over them. And it is worth asking how the devil managed to gain power over them at that day.
The answer is that he got power over them while they were here in mortality. It happens especially to those who violated the most serious commandments to not kill and to not commit adultery. The devil only has power over man after this life by gaining power over that man here in mortality. And he only gains power over them in mortality through sin.
Any man who has subjected his own flesh to the laws of God should know in his heart that a man not bound by the same laws will not choose to keep them. Mere "ethics" will never keep man morally clean. And as mere ethics will not keep a man morally clean, mere ethics will consequently deliver a man over to the power and sway of the adversary. And then they will fall prey to all manner of mischief. You cannot give such powerful sway to the adversary through serious sin without feeling the weight of temptation and influence he will bring to bear once you have.
Man needs commandments from God.
Ethics is just the gateway to moral sin while maintaining a show of goodness.
And moral sin leads down the path to depravity. It may be depravity with the trappings of civilization and polite society still intact. But you cannot break the great commandments repeatedly and not find yourself descending in time to the darkest recesses of the pit.