Allah reminded the Children of Israel of the pledges, covenants and promises that He took from them to believe in Him alone, without a partner, and follow His Messengers. Allah stated that when He took their pledge from them, He raised the mountain above their heads, so that they affirm the pledge that they gave Allah and abide by it with sincerity and seriousness. Hence, Allah's statement,
(And (remember) when We raised the mountain over them as if it had been a canopy, and they thought that it was going to fall on them. (We said): "Hold firmly to what We have given you (Tawrah), and remember that which is therein (act on its commandments), so that you may fear Allah and obey Him.") (7:171).
The mount mentioned here is At-Tur, just as it was explained in Surat Al-A`raf, according to the Tafsir of Ibn `Abbas, Mujahid, `Ata', `Ikrimah, Al-Hasan, Ad-Dahhak, Ar-Rabi` bin Anas and others. This is more obvious. There is another report from Ibn `Abbas saying; `The Tur is a type of mountain that vegetation grows on, if no vegetation grows on it, it is not called Tur.' And in the Hadith about the trials, Ibn `Abbas said; "When they (the Jews) refused to obey, Allah raised the mountain above their heads so that they would listen."
Al-Hasan said that Allah's statement,
(Hold fast to that which We have given you) means, the Tawrah. Mujahid said that the Ayah commanded, "Strictly adhere to it." Abu Al-`Aliyah and Ar-Rabi` said that,
(and remember that which is therein) means, "Read the Tawrah and implement it." Allah's statement,
(Then after that you turned away. Had it not been for the grace of Allah) means, "Yet, after the firm pledge that you gave, you still deviated and broke your pledge";
(Had it not been for the grace and mercy of Allah upon you), meaning, by forgiving you and by sending the Prophets and Messengers to you,
(Indeed you would have been among the losers) meaning, in this life and the Hereafter due to their breach of the covenant.