Was Ka'b al-Ahbar a Jew who feigned Islam?

Ka'b al-Ahbar was a Muslim, and many of the Salaf praised and commended him. Ibn Sa'd said: They mentioned him to Abu al-Darda, who said: "Indeed, the son of the Himyariya possesses much knowledge." Ibn Sa'd also narrated, through Abd al-Rahman ibn Jubayr ibn Nufayr, that Mu'awiyah said: "Indeed, Ka'b al-Ahbar is one of the learned scholars; his knowledge is like the seas, and we are indeed negligent in it." Al-Hafiz Ibn Hajar said of him in Al-Fath: "He was one of the best of the rabbis." Imam al-Dhahabi included his biography in Tadhkirat al-Huffaz and Siyar A'lam al-Nubala', and Ibn Asakir extensively covered his biography in Tarikh Dimashq. Abu Nu'aym also elaborated on him in Hilyat al-Awliya', and none of them impugned his religiosity.