a disbelief in the existence of deity
An atheist doesn't believe in a supreme being or God or a spiritual existence of any kind. They believe you're born, you live and then you die and that's it. It's a very tidy and convenient existence. You don't have to go to church or pray or take communion or give money or read bibles or talk to angles or anything like that.
So they get rid of the religious stuff, which creates a void in their thinking, and before lone it starts filling in with all the fleshy trash that makes up the secular world: anger, less work, more sex, drugs, envy, jealousy, gay marriages and just about anything that's contrary to God. Life is good for a while, at least until they get bored and start thinking about what's next. But there is nothing, right! Humm, life's getting short now.
They know if there is another life, it'll be a spiritual one that includes a God and they know better than to take that route again. Their not going to give up their fleshy freedom and besides they'd rather be dead than admit sins, especially to a stranger. That's right! Better dead than humble! And they also have to protect your stinking pride until their last breath. Satan must love them deeply.
Do they know that God also loves them deeply and is willing to forgive all who believe in him!
"Before everyone are life and death
whichever they choose will be given them." Sir 15:17.