You should know that people who deny that God exists do know that God exists.  How can you know that?  God reveals it to us through Scripture.  God can make such a claim since He is God and knows all things.

God says,

“For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse.” Romans 1:20

God is just, so He must judge sin.  The worst sin is the sin against the most important commandment. 

Mark 12:29-31 And Jesus answered him, The first of all the commandments is, Hear, O Israel; The Lord our God is one Lord: And thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind, and with all thy strength: this is the first commandment. And the second corresponds, namely this, Thou shalt love thy neighbor as thyself. There is none other commandment greater than these."

The Sin:

Romans 1:18 For the wrath of God is unveiled from heaven against all ungodliness and unrighteousness of men, who restrain the truth in poor judgment, * because that which may be known of God is plainly known among them, for God has made it visible it to them. Note  For the invisible things of Him from the creation of the world are clearly seen, being understood by the things that are made, even His eternal power and Godhead, so that they are without excuse, because, when they knew God, they didn't glorify Him as God, neither were they thankful, but they became vain in their imaginations, and their foolish innermost minds were darkened.

The Punishment:

Romans 1:24-32 Therefore God also gave them up into uncleanness through the desire for forbidden things that is within their own innermost minds, to dishonor their own bodies between themselves . . . 26 For this cause God gave them up to vile afflictions of the mind, for even their women changed the natural use into that which is against nature, And in the same way the men also, leaving the natural use of the woman, burned in their desire one toward another, men with men, committing indecent acts and receiving in themselves that penalty of their error that is just. And even as they did not like to retain God in their knowledge, God gave them over into their depraved minds to do those things which should never be done. * Being filled with all unrighteousness, fornication, wickedness, covetousness, desire to harm; full of envy, murder, debate, deceit, depravity; slanderers, Backbiters, haters of God, spiteful, proud, boasters, inventors of evil things, disobedient to parents, Without understanding, covenant breakers, without natural affection, implacable, unmerciful: Who knowing the judgment of God, that they which commit such things are worthy of death, not only do the same, but have pleasure in those who do them.

As you can see, all the other sins are the result of the first sin.  Honoring God is the beginning of wisdom.  Wisdom consists of righteousness, holiness, and being set free from sin's bondage.  And all of these sins must be judged by God--and the ungodly people (Atheists and all who do not acknowledge God) actually know that they will be judged.  But they are defiant.  Now, just because a person wears the name, Christian, doesn’t exclude them from inclusion in this group.  God goes on to say in Proverbs 3: 5-8 Trust in Jehovah with all your mind; and lean not to your own understanding.  In all your ways acknowledge him, and he shall direct your paths. * Be not wise in your own eyes: reverence Jehovah, and depart from evil. It shall be health to your navel, and marrow to your bones. *  So, this verse also applies to all religious people who don’t listen to God’s voice and take His direction.  Everyone hears God’s Voice.  God reveals that clearly though Scripture.  But who will acknowledge His Voice?

Judgment will not be equal for everyone.  Think of the Atheist who leads his or her family into not acknowledging God.  That person’s judgment will be much more severe than the Atheist who just quietly fails to acknowledge God’s leading.  It will be much worse for the Atheist who goes out of his or her way to try to cause Christ-followers to fall.  You may wonder how any person could be so evil.  These are people who have given themselves over to Satan to follow Satan with their whole heart, soul, and mind.