The self-proclaimed atheist is a self-claimed absolute relativist,...therefore, 'to know' for him is dis-allowed.
He'll find out about 'know' in hell if he doesn't trust/believe the 'Truth' of his saviour Jesus Christ.
St. John 14:6
Does that mean all non-Christians go to hell?
While I will not defend atheism, it is a bit naive to think that moral rules can only be known by divine revelation and further, that atheists are by definition, moral relativists. That is simply not true. One can observe human behavior and derive a perfectly sound set of rules and values to govern social groups.
For example: Thou Shall Not Kill is pretty obvious to most people. If you want to get along in a group it is simply wise to make an agreement not to kill each other and punish those who violate the rule.
BTW, I would not give you as strong an argument if you had said that most moral relativists are atheists (at least at heart). That is much closer to the truth. The opposite is simply not true. There are many moral, upstanding, responsible, Christian Law abiding atheists.
Religion resulted from mans first feeble attempts to control and understand the world around him. The basic premise is that God controls the world and that if you please him, he will in turn do good things for you. The rules (morals) were to obey and serve God and his representatives.
As religions became more sophisticated they developed societal rules that helped reinforce the religion and society and made them stronger. Those rules became morals, the rules that didn't work were dropped by the wayside.
Morals are simply a balancing act between immediate gratification and long term happiness. Once that simple fact is understood, living a moral, joyful life is as easy as floating down the river. It probably doesn't appeal to Christians who enjoy beating themselves up and resisting temptation and repenting after they sin, though.