We must believe in the existence of God, inasmuch as we believe in our own. Living being exist and so does God Who is the sum-total of all life.
By a disbelief in God we injure ourselves in the same way as if we disbelieved in ourselves. That is to say, not to believe in God is something like suicide. But it is one thing to believe in God, and quite another thing to conduct ourselves as if we had such a living faith. As a matter of fact there is no atheist in the world; atheism is a mere pretence.