We can't believe God if we have a stubborn heart. A stubborn heart is always unreceptive to the things of God. The reason why Pharaoh didn't adhere to the word of God was because his heart was unreceptive.
Nothing is more important than knowing the One who created us and gives our lives meaning. Even when we cannot see Him physically, we can't deny the fact that we can see His works all around us. Starting with the beauty of creation, how everything is properly arranged and ordered in the universe, or the breath in our lungs that gets exhausted.
Psalm 19:1 says, “The heavens declare the glory of God.”
Even the works of creation announce the m existence of God. Another reason we should believe in God's existence is because of His faithfulness. We may not be aware but there were multiple times in life where He guided us and shielded us from evil.