While atheism does not have any doctrine of its own, it's simply the lack of a belief in gods, there are doctrine-like philosophical consequences of not having a belief in gods.
Perhaps the primary consequence of not believing in gods, and the one that theists find the most repugnant, is the conclusion that there is nothing after death; no everlasting life (up above or down below), no karma, no reincarnation. To not believe in gods and the religious beliefs associated with the various systems of faith means that we therefor believe that there's nothing after this life, nothing. Our cells quit functioning along with our internal organs, synapsis stop firing in the brain and our flesh begins to loose coherence and starts to decompose. We cease to be. We return to a non-existence as real as the non-existence that was our state before our conception.
Beliefs in an afterlife offers consolation and comfort to those who have lost loved ones and assurance to those who fear the finality of death. But like the belief in god, the desire or need to believe in life after death does not mean that such a thing has to exist.
Atheists and others who accept that there's no credible evidence that anything exists beyond death that we'll be able to appreciate or comprehend are left having to conclude that this life is all we get, a one-shot deal. No matter how long or short, no matter how happy or sad, no matter how profitable or misspent, this life is the only life we'll have.
There are many lessons I learned as a theist years ago that stay with me, that still have relevance to my current non-believing life. One of them is what Jesus is reported to have said in Matthew 6:34, "Therefore do not worry about tomorrow, for tomorrow will worry about itself. Each day has enough trouble of its own."
For the non-believer, each day, every hour, each moment, is a gift from the universe. We will never have another chance to live this minute, to experience this moment. Worrying about the future or the past, while inevitable for most humans, is also a waste of time, the most precious thing we all posses. We understand that we may never have another moment to express our love for someone, we may never get another chance to show mercy or perform an act of kindness. Most of us will never know exactly when this life will come to an end. It may be years from now, after you've lived a full and gratifying life, had children and grandchildren, achieved the majority of goals you set for yourself. Or it may come to an end in the next hour, the next minute. An organ may fail or a car crash into yours. Your life could end before you ever have a chance to mend a broken relationship, to experience something you've longed to do for years. Acknowledging that fact should compel you to treat every moment as if it was your last, to try and squeeze the most enjoyment, the greatest good, the most benefit from every second of your life. No little bit of awareness should pass unnoticed, no moment of consciousness pass unappreciated.
If this is the only chance we'll ever get to experience this thing we call life we cannot justify wasting any second of it. If death is the final act in this play of life then we cannot justify missing a moment of this life distracted by the unlikely possibility of a life to come after we die.