Alleged Contradictions in the Qur'an answer
Abraham committed this sin of polytheism as he takes moon, sun, stars to be his Lord [6:76-78], yet Muslims believe that all prophets are without any sin.
Verses in question:
6:76-78. When the night covered him over with darkness he saw a star. He said: "This is my lord." But when it set, he said: "I like not those that set."
When he saw the moon rising up, he said: "This is my lord." But when it set, he said: "Unless my Lord guides me, I shall surely be among the erring people."
When he saw the sun rising up, he said: "This is my lord. This is greater." But when it set, he said: "O my people! I am indeed free from all that you join as partners in worship with Allâh.

1. The general interpretation of the above verses is that this was an act done by Prophet Abraham to lead others to Islam. He had travelled to a land where people worshipped celestial bodies, and through this play, he demonstrated to them that only Allah is worthy of worship. He did not actually believe that these objects were His lord, but he wad pretending in order to drive home a point to his people in a special way. As Ibn Kathir Ad-Damishqi (d. 1372CE) writes in his renowned Tafsir Al-Qur'an Al-Azim:
We should note here that, in these Ayat, Abraham, peace be upon him, was debating with his people, explaining to them the error of their way in worshipping idols and images....When he proved that these three objects were not gods, although they are the brightest objects the eyes can see, he said: "O my people! I am indeed free from all that you join as partners in worship with Allah.'' meaning, I am free from worshipping these objects and from taking them as protectors. Therefore, if they are indeed gods as you claim, then all of you bring your plot against me and do not give me respite.(Tafsir Ibn Kathir, Abridged, Darussalam Publishers & Distributors, 2000, vol. 3, pp. 389-390)
2. Even if we suppose that Abraham was not pretending, this still would not prove the claim. This event occured before Abraham became a Prophet, and furthermore, there is no evidence that he was worshipping the celestial bodies, but only pondering on what is truly divine.

3. While we are on the subject, we shall comment on the Islamic view on Prophets. Prophets are human beings who are selected to recieve revelation from Allah and guide people to the straught path. They are the best of humanity, so that their followers may take them as role models. Nevertheless, they are human, and thus, they are sometimes subject to a temporary lapse in judgement or minor mistakes. But they are believed to be free of sins, or infallible, especially after recieving revelation from Allah.
Shaykh-ul-Islam Ibn Taymiyah (d. 1328 CE) said:
The view that the Prophets are infallible and protected against committing major sins, as opposed to minor mistakes, is the view of the majority of Muslim scholars and of all groups. It is also the view of the scholars of tafseer and hadeeth and fuqaha’. Indeed, nothing has been narrated from any of the salaf, imams, Sahaabah, Taabi’een and those who followed them except that which is in accordance with this view. (Majmoo’ al-Fataawa, 4/319)