Beliefe in God in Buddhism is a choice. A Buddhist can believe in God.
Siddhārtha Gautama was a Hindu reformist. He believed that it had become fettered. Theology, ritual, authority, grace, mystery and tradition are components of religion. He believed they had become overbalanced. Where the form had replaced the meaning. He didn't reject them. He rejected the imbalance. He was a called the rebel child of Hinduism. He asked his followers, "did I ever teach you that God existed?" They said, "no." He asked them, "did I ever teach you that God didn't exist?" They said, "no." He asked them, "did I teach you the world is eternal?" They said, "no." He asked them, "did I teach you the world isn't eternal?" They said, "no." He then asked them, "what have I taught you?" They said, you have taught us suffering and the end of suffering."Belief in God is not required in Buddhism, but may be practiced.
He was a rationalist and a moralist. He taught that people should work out their own salvation with diligence. He was not against Hinduism. He was against the corruption which had entered into Hinduism and wanted to get back to what was important in Hinduism. He believed we crave and cling to impermanent states and things which are incapable of satisfying us. Which is very true. We are free to pursue pleasure, wealth, fame and power but none of those things will satisfy us because we were made for more.