If Buddha did not believe in the existence of God, then what was the central theme of Buddha's preaching?

Although Buddha did not believe in the existence of God, the central theme of his preaching was to help people end suffering and attain peace and happiness through the Four-Noble truths and the Eight-fold path. His teachings were based on ahimsa or non-violence, love and compassion for all living beings. The Buddha urged his followers to avoid living a life of extremes and to follow the Middle Path.