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 )