Ibn Mandah (d. 395 H) says in his book “At-Tawheed”:
Chapter: Evidence that a person who does ijtihad but errs regarding knowing Allah ‘Azz wa Jall and His Oneness is akin to one who is obstinate. Allah the Exalted says, describing their misguidance and obstinacy: ﴾Say, ‘Shall We inform you of the greatest losers as to [their] deeds? [They are] Those whose efforts have been wasted in this life, while they thought that they were acquiring good by their deeds!’﴿ [Al-Kahf: 103-104].
When Ali ibn Abi Talib (may Allah be pleased with him) was asked about those who are “greatest in loss regarding [their] deeds,” he said: “They are the disbelievers among the People of the Book. Their predecessors were upon the truth, but they associated partners with their Lord—Mighty and Majestic is He—introduced innovations into their religion, and brought about changes [contrary to the truth] upon themselves. Thus, they unite in error while thinking they are rightly guided, and they strive in falsehood while thinking they are upon the truth; their efforts in this worldly life have gone astray, even as they imagine they are doing good work.” Ali (may Allah be pleased with him) added that the people of Harura’ (Khawarij) are among them.
149 – Muhammad ibn al-Husayn ibn al-Hasan informed us, saying: Ahmad ibn Yusuf informed us, Abd al-Razzaq informed us, from Ma‘mar, from Hammam ibn Munabbih, who said: This is what Abu Hurayrah narrated to us, saying: The Messenger of Allah (peace and blessings of Allah be upon him) said: “By the One in Whose Hand is my soul, no man of this Ummah—nor a Jew, nor a Christian—hears of me and then does not believe in me, but that he will be among the people of the Fire.”
150 – Abu al-Tahir Ahmad ibn ‘Amr informed us, saying: Yunus ibn ‘Abd al-A‘la narrated to us; al-Hasan ibn Wahb informed us; ‘Amr ibn al-Harith informed us, from Abu Yunus, from Abu Hurayrah—may Allah be pleased with him—from the Messenger of Allah (peace and blessings of Allah be upon him), who said: “By the One in Whose Hand is my soul, there is no one of this nation—whether a Jew or a Christian—who hears of me, dies, and does not believe in that with which I was sent, but that he will be among the people of the Fire.”
151 – Muhammad ibn al-Husayn informed us—Ahmad ibn Muhammad ibn Nasr narrated to us—and Ahmad ibn Muhammad ibn ‘Asim informed us, saying: ‘Abdullah ibn Muhammad ibn al-Nu‘man narrated to us—[both] said: ‘Amr ibn Hammad narrated to us, Asbat ibn Nasr narrated to us, from Isma‘il al-Suddi, from Abu Malik and Abu Salih, from Ibn ‘Abbas, and from Murrah, from ‘Abdullah ibn Mas‘ud—that Salman al-Farisi (may Allah be pleased with them all), while speaking with the Prophet (peace and blessings of Allah be upon him), mentioned his companions to him and told him about them, saying: “They used to fast, pray, and bear witness that you would be sent as a Prophet.” When Salman finished praising them, the Prophet (peace and blessings of Allah be upon him) said: “O Salman, they are among the people of the Fire.” This weighed heavily on Salman, for he had previously told the Prophet: “Had they lived to see you, they would have believed in you and followed you.” Consequently, Allah—Mighty and Majestic is He—revealed this verse: {Indeed, those who believed—and those who were Jews, the Christians, and the Sabians—whoever believed in Allah and the Last Day…}. So Iman for the Jews was to hold fast to the Torah and the way of Moses until the coming of Jesus; however, once Jesus (peace be upon him) arrived, anyone who clung to the Torah and the way of Moses—without abandoning them or following Jesus—was doomed. As for the Christians, Iman for them was that whoever among them held fast to the Gospel and the laws of Jesus was a believer whose faith was accepted; however, when Muhammad (peace and blessings of Allah be upon him) came, anyone who did not follow him—and did not forsake the way of Jesus and the Gospel they had previously adhered to—was doomed.
At-Tawheed by Ibn Mandah, pg 261-262