Assuming that the stories in the Hebrew Bible are true, then the people who are depicted therein knew that God was real. For them, God was a matter of indisputable fact, because he performed miracles via his angels and prophets, such as having his angel speak to Moses from a burning bush, inflicting ten plagues upon Egypt, and having his angel lead the Israelites in the desert and providing them with mana from heaven. However, for people who are alive today, God does not perform miracles but instead supposedly requires us to to have faith in the stories in which he supposedly did so multiple millennia ago.
Where is the fairness in that? Why should anyone who is alive today believe that God exists when people who supposedly lived thousands of years ago were provided with proof?