Overall meaning :
Allah Almighty tells His Prophet, may Allah bless him and grant him peace, that the People of the Book—the Jews and Christians—will not be pleased with him until he follows their religion, because they consider themselves are rightly-guided, so Allah Almighty teaches him to refute such a false claim by saying to them that Allah’s guidance is the only true guidance which Allah, Glorified and Exalted be He, has given to him.
Then He enjoins His Prophet, may Allah bless him and grant him peace, and by extension all his followers, not to follow the desires of the People of the Book after Allah has given him the knowledge that shows him the truth, stating that if he did follow their desires, he would then find neither helper to do him any good, nor protector to keep him out of harm’s way.
Next, the Almighty states that those who dutifully follow their book from among the People of the Book—by not distorting it and making any changes in it in any way, among other things—prove that they truly follow it, and this will surely pave the way for them to believe in Muhammad, may Allah bless him and grant him peace, for their books acknowledge him as a prophet and mention his description as well. As for those among them who disbelieve their book—by perverting it, making changes in it and obstinately denying Muhammad’s prophethood mentioned in it, among other things—these will surely be the losers.
Then, addressing the Children of Israel, Allah Almighty reminds them of His blessings which He has bestowed upon them, by which He means the blessings He bestowed upon their forefathers, adding that He preferred them over all the nations of their time. After reminding them of the favours He conferred upon them, He warns them and admonishes them to fear the punishment of the Day of Judgement, on which no soul will pay on behalf of another, nor will any ransom be accepted from it, nor will intercession be accepted on anyone’s behalf if it does not profess faith, nor will anyone save them from the punishment of Allah Almighty.