The Shaykh (may Allah have mercy on him) said:

The third nullifier: Whoever does not regard the polytheists as unbelievers, or entertains doubt about their unbelief, or approves of their creed, has fallen into unbelief.

“The third” among the nullifiers is: “whoever does not declare the idol-worshippers—those who worship alongside Allah anything else—to be unbelievers.” They bow to stones and trees, or to the dead, or to cows, or to the cross, or to the Messiah and his mother, just as Allah, exalted is He, says: “And [beware the Day] when Allah will say, ‘O Jesus, son of Mary, did you say to the people, “Take me and my mother as deities besides Allah”?’” (Surah al-Māʾidah 5:116). He, exalted is He, also says: “They have certainly disbelieved who say, ‘Allah is the Messiah, the son of Mary’” (Surah al-Māʾidah 5:17). Thus, whoever does not declare such people to be unbelievers is himself an unbeliever. An example is someone who claims that the Jews and Christians are upon a sound religion—or, as some sects even say, that the idol-worshippers are upon the truth and that their religion is valid! Whoever fails to pronounce unbelief upon those whom Allah and His Messenger have declared unbelievers has committed unbelief.

His words, “or is uncertain about their unbelief,” likewise constitute unbelief, because doubting the truth is tantamount to denying it. For instance, a person might say, “By Allah, we do not know whether the Jews and Christians are upon the truth or not,” or he might say, “Each person may follow whatever religion suits him.”

His words, “or approves of their creed,” apply to someone who says, “They are on a sound religion, and the paths to Allah are diverse; just as the Muslims are on a sound religion, so are they,” or who says, “Their religion is correct in their own eyes, just as Islam is correct in the eyes of the Muslims.” Whoever says this must be shown that his statement is false and that he has not