This is a classic question and has been explored by many scholars and laymen, theists and atheists.
Modern atheists or anti-theists believe God was an explanation of nature and the universe when we didn't know enough
about science. With today's science, the concept of God can be explained away. In other words, with science, we can now explain everything with God.
But science cannot explain everything. For example, science does not explain morality. Why do scientists need to be honest in publishing their scientific results?
Without God, it is actually pretty hard to explain the existence of evil and good, which is evident. The very fact of the existence of evil and good tells us that there are moral rules that are applied, either explicitly or implicitly. Therefore,
there exists the creator of moral rules, which we refer to as God.
In our discussion, God is the first cause of everything. The question then becomes, if God exists, who is believed to be good and all-powerful, how can evil exist?