Let's start with a very basic definition of God: Almighty.
What does this imply? That He is the creator, the one who is most supreme, has control over all beings and non-beings.
Let's add to it what we like to believe (or rather already do believe): That He is the doer of good and will always make truth / good triumph over falsehood / evil.
And finally, let's add a reason for our belief: That whenever we are in times of crisis, He is the one we can always rely on to help us, to set things right, to get us out of our misery and lead us onto the right path.
Having said that, our definition of God today seems to have merged to include even our beliefs. Let me explain this better with a couple of examples.
The origin of religion actually began when man started worshiping the things that helped him survive - like fire and Sun and earth. Then he started worshiping the things that he felt could cause some disruption to him - for he believed that by praying to these elements he can pacify them to cause less destruction - such as praying to wind and rain to harness and not destroy his crops.
He slowly started assigning entities to each of these elements, entities that control these elements. We see examples of this across several civilizations. Let me just state an example of the Wind God. He is is Aeolus in Greek mythology, Vayu in Hindu mythology, Shu in Egyptian mythology, Fujin in Japanese mythology, Fei Lian in Chinese mythology and so on so forth.
Once man had the entities in place, he began to observe and study in deep the factors that impact these elements. He started devising ways to appease the Gods. These later became the religious rituals. It gradually began to be believed that those practicing the rituals were to have a good life (good crop, good children, good health and so on - for each aspect of life had a God who controlled these aspects).
One such God that man believed in was the God of Justice. And while the concept of Justice was completely abstract and "defined" by man himself and not "observed" as in case of element Gods, the concept was so powerful that it gained instant popularity (or let's say in this case "followers"). Religion was defined as following the path of Justice and righteousness.
This is where the problem begins.
While we may be able to validate the impact of actions of the element Gods, what we cannot validate are the actions of the abstract Gods like the God of Justice. If our belief in God assumes him to do good to all, then why does bad happen to good people? Why is there so much evil still prevailing in the world?
Such imbalances makes one ponder whether whether God does infact exist. If so, then does he have as much power as we believe him to have? If yes, then does he also use this power in ways that we have convinced ourselves of - to bestow justice upon all?
Simply put, there is simply no proof of action of God and hence, the question remains open - does God exist?
Let me put it in the words of the famous Greek philosopher Epicurus who states this very straightforwardly through his trilemma argument:
"Is God willing to prevent evil, but not able? Then he is impotent. Is he able, but not willing? Then he is malevolent. Is he both able and willing? Then whence cometh evil? Is he neither able nor willing? Then why call him God?"
