Abraham proved to people the fact that there is no god worthy of worship but Allah

Allah showed Abraham the way of realizing the secrets of the universe, in order to infer by this the oneness and greatness of the creator.

Allah said ( interpretation of the meaning ) : { And thus did We show Abraham the realm of the heavens and the earth that he would be among the certain [in faith]. So when the night covered him [with darkness], he saw a star. He said, "This is my lord."1 But when it set, he said, "I like not those that set [i.e., disappear]." And when he saw the moon rising, he said, "This is my lord." But when it set, he said, "Unless my Lord guides me, I will surely be among the people gone astray." And when he saw the sun rising, he said, "This is my lord; this is greater." But when it set, he said, "O my people, indeed I am free from what you associate with Allah. Indeed, I have turned my face [i.e., self] toward He who created the heavens and the earth, inclining toward truth, and I am not of those who associate others with Allah." } Quran ( 6 : 75 - 79 )