And therefore, at the event of Har Sinai, Hashem said: "I am Hashem your God, who brought you out of the land of Egypt" (Shemos 20:2), and not 'who created the world,' because Egypt already believed that Hashem created the world. However, the purpose of Yetziyas Mitzrayim was to reveal the belief in Hashem's guidance and providence in His world, and the first commandment was directed against this belief.