Tafsir of Ar-Ra'd 13:36

{And those to whom We have given the Book} He means those who accepted Islam from among the Jews, such as ʿAbd Allāh ibn Salām, Kaʿb, and their companions; and those who accepted Islam from among the Christians, who were eighty men: forty from Najrān, thirty-two from the land of Abyssinia, and eight from the people of Yemen.

{rejoice in what has been revealed to you, and among the factions are those who deny some of it} He means among their factions—those who disbelieved and banded together in enmity against the Messenger of Allah (ﷺ), such as Kaʿb ibn al-Ashraf and his companions, and al-Sayyid and al-ʿĀqib, the two bishops of Najrān, and their followers. They did not deny the narratives, some of the rulings, and the meanings that were established in their books without alteration. Rather, they denied the descriptions of Islam and the descriptions of the Messenger of Allah (ﷺ), and other matters they had altered and changed from the laws.

If you ask: How does the statement {Say: "I have only been commanded to worship Allah"} connect to what precedes it? I say: It is a response to the deniers. Its meaning is: Say, "I have only been commanded in what has been revealed to me to worship Allah and not associate anything with Him." Your denial of it is a denial of the worship of Allah and His Oneness. So look at what you are denying, while you claim the necessity of worshipping Allah and not associating anything with Him, as in the verse: {Say, "O People of the Scripture, come to a word that is equitable between us and you - that we will not worship except Allah and not associate anything with Him"} (Āl ʿImrān: 64).

Nāfiʿ read—in the narration of Abū Khulayd—{wa lā ushriku} (and I do not associate) in the nominative case, as an independent clause, as if he said: "And I do not associate anything with Him." It is also permissible for it to be in the state of a circumstantial qualifier (ḥāl), meaning: "I have been commanded to worship Allah, not associating anything with Him."

{To Him I invite} Specifically; I do not invite to anyone else.

{and to Him} Not to anyone else, is my return. You say the same, so there is no meaning to your denial.

{And thus We have revealed it as an Arabic judgment. And if you should follow their inclinations after what has come to you of knowledge, you would not have against Allah any protector or any defender.}