Overall meaning : Allah, commands His Prophet, may Allah bless him and grant him peace, to invite the People of the Book (the Jews and the Christians) to a fair and equitable agreement involving them all: to declare Allah’s oneness, to worship none besides Him and not to take any lords to be worshipped besides or instead of Allah or to be obeyed, instead of Allah, in what they declare lawful or unlawful. If they obstinately refuse to do so, He urges the believers to call them to witness that they have yielded themselves in absolute surrender to Allah and that they will persistently continue to profess their religion which Allah has chosen for them and to follow the laws that He has laid down in it.
Then He addresses the Jews and the Christians thus: People of the Book, why do you argue about Abraham—each of you claiming that you follow his religion—when both the Torah and the Gospel were not revealed until long after his demise? Do you not, then, realise that you are only telling falsehoods? You have argued regarding what you know about your religion and what is mentioned in your books. Must you now argue about Abraham and the religion he followed when no knowledge regarding the matter has reached you?! Allah’s knowledge encompasses everything that is completely unknown to you, and you surely do not know what He knows!
Allah then points out to them the clear truth about Abraham and the true nature of the faith He revealed to him, categorically stating that Abraham was neither a Jew nor a Christian, but an upright prophet who surrendered himself to Allah, was wholly devoted to Him and sincerely worshipped Him, and that he never associated partners with Him by worshipping idols or other false gods, for that matter, besides Him.
After that, He mentions those who have the strongest claim to Abraham and who are truly the closest to him and support him, stating that they are those who practise his religion and follow in his footsteps, including Prophet Muhammad, may Allah bless him and grant him peace, and all those who believe him and follow his example. These are, indeed, the ones who have the best claim to Abraham and who truly support him, and Allah is the guardian and protector of the believers, whom He will surely support against those who oppose them.