We can only know God is there if he chooses to make himself known. He has done this by showing something of himself in the natural world, revealing himself in the history of Israel, speaking in the Bible, and supremely in the person of Jesus who life is recorded there.
Had we lived almost 2000 years ago, we could have seen God face to face. Jesus claimed to be God and he backed up those claims with what he said and did. He calmed storms, healed the sick, cast out evil spirits, raised the dead, and forgave people. He lived the most perfect life, and gave us the most profound teaching. When Jesus himself was put to death, he rose again. The evidence of what Jesus' said and did therefore demonstrate that he was who he claimed to be. The gospel writers had nothing to gain by making it up. And the truth of what they wrote is confirmed by its impact. The way his teaching continues to transform lives today can be explained no other way.(Romans 1:18-20, John 5:16-42)