Abbas - Tanwîr al-Miqbâs min Tafsîr Ibn ‘Abbâs
Allah mentioned after this the dispute of the Jews and Christians with the believers, saying: (And they say) i.e. the Jews: (Be Jews) you shall be guided from error (or Christians) and the Christians also say the same, (then ye will be rightly guided. Say) to them, O Muhammad: it is not as you claim: (Nay, but the religion of Abraham, the upright) the Muslim; rather follow the religion of Abraham, the upright, the Muslim, the sincere-only then you will be guided, (and he was not of the idolaters) i.e. he did not follow the religion of the Jews or Christians.
And they say, ‘Be Jews or Christians (the particle aw is for detail; the first of these is the saying of the Medinan Jews, while the second is that of the Christians of Najrān), and you shall be guided’. Say, to them: ‘Nay, we follow, rather the creed of Abraham, a hanīf (hanīfan is a circumstantial qualifier referring to Ibrāhīma, that is to say, one that inclines away from all other religions to the upright religion); and he was not of the idolaters’.
And they say, 'Be Jews or Christians [and you shall be guided']: every person who is veiled by his religion claims that his religion and none other is the truth. Say, 'Nay, rather the creed of Abraham [a ḥanīf; and he was not of the idolaters'], for absolute guidance is the affirmation of God's Oneness which comprehends every religion and lifts every veil, as He says next in His statement:
Muhammad bin Ishaq reported that Ibn `Abbas said that `Abdullah bin Suriya Al-A`war said to the Messenger of Allah, "The guidance is only what we (Jews) follow. Therefore, follow us, O Muhammad, and you will be rightly guided.'' Also, the Christians said similarly, so Allah revealed,
(And they say, "Be Jews or Christians, then you will be guided.'') Allah's statement,
قُلْ بَلْ مِلَّةَ إِبْرَهِيمَ حَنِيفًا
(Say (to them O Muhammad ), "Nay, (we follow) only the religion of Ibrahim, Hanif) means, "We do not need the Judaism or Christianity that you call us to, rather,
مِلَّةَ إِبْرَهِيمَ حَنِيفاً
((we follow) only the religion of Ibrahim, Hanif) meaning, on the straight path, as Muhammad bin Ka`b Al-Qurazi and `Isa bin Jariyah stated. Also, Abu Qilabah said, "The Hanif is what the Messengers, from beginning to end, believed in.'')
Maududi - Sayyid Abul Ala Maududi - Tafhim al-Qur'an
In order to understand the true significance of this answer, two things should be kept in mind.
(1) Judaism and Christianity were born long after the death of Abraham. Judaism, with its special rituals, peculiar regulations, etc., took its birth and name some tour hundred years before Christ. As to "Christianity", it took its name and adopted its special creed and form long after the recall of Jesus. Thus it is clear that their claim that one must become a Jew or a Christian in order to obtain guidance was historically untenable. For in that case, Abraham, Jesus and all the other Prophets and all the good people who had passed away long before the birth of Judaism and Christianity could not be counted among the right-guided persons for the simple reason that these "religions" did not exist at the time they lived. Thus, it was obvious that the Jews and Christians could not say that these Prophets were not right-guided, nor could they claim that they followed Judaism or Christianity. As a corollary to this, true Guidance did not, even according to them, consist of the particular features which divided them into two distinct religions, but it was that eternal universal way which has been shown by all the Messengers of Allah and which has always been followed by the good people of all ages.
(2) It also meant to warn the Jews and the Christians that both practised shirk and had, therefore, swerved from the way of Abraham, who did not associate any other object with Allah in his worship, his reverence, his submission and his obedience to Him. They could not deny this because their own Scriptures bore testimony to it.
"We do not discriminate against any of them" means that we believe in all the Prophets of Allah and reject none. It is obvious that all the Prophets who came from Allah brought the same Truth and invited the people to the same Guidance and Right Way. Hence anyone who follows the Truth must accept all the Prophets. Those who accept one Prophet and reject the other really reject even the one whom they profess to accept. Had they followed the universal Guidance taught by Moses, Jesus and other Prophets, they could not have rejected any other Prophet (i.e. Muhammad, Allah's peace be upon him). As a matter of fact, they were not following the teachings of any Prophet, but merely professing to accept their own Prophets just because their fathers did so. Thus their real religion was prejudice, race-worship and blind imitation of their forefathers.
And they say “Be Jews or Christians and you will be guided.” Say “Nay rather the creed of Abraham a ḥanīf; and he was not of the idolaters.” [That is] “When the groups contended with you [Muḥammad] they demanded that you agree muwāfaqa with them. Stand firm in opposing that to which they call you and turn your attention increasingly to Us. Follow the way of the Friend al-khalīl in withdrawing from the group whether it be one's own father or anyone else who is not in accord lā yuwāfiqu with his Protector.” Thus he said “And I shall shun you and that which you call upon besides God” [19:48]. The ḥanīf is the “one who leans” toward the straight truth on the path the truth free from any defect of creation. [He is] in agreement wāfiq with the truth by the Truth.
(And they say: Be Jews or Christians, then ye will be rightly guided…) [2:135]. Said Ibn ‘Abbas: “This was revealed about the Jewish leaders of Medina: Ka‘b ibn al-Ashraf, Malik ibn al-Sayf, Wahb ibn Yahudha, Yasir ibn Akhtab, and also about the Christians of Najran. This is because when they argued with the Muslims regarding religion, each one claimed to have a better right to the religion of Allah, exalted is He, than anyone else. The Jews said: ‘Our Prophet Moses is the best of prophets, and our Scripture is the best of Scriptures and our religion is the best of religions’. They disbelieved in Jesus, the Gospel, Muhammad and the Qur’an. The Christians on the other hand said: ‘Our Prophet Jesus is the best of prophets, and our Scripture, the Gospel, is the best of Scriptures and our religion is the best of religions’. They disbelieved in Muhammad and the Qur’an. Both the Jews and Christians said to the believers: ‘Follow our religion, for there is no other true religion except ours’. And they invited the believers to follow their religion”.