What is the ruling on one who says: "I do not declare polytheists to be disbelievers unless I know that they are disbelievers or polytheists," and is this action considered one of the nullifiers of Islam?

A Muslim must believe in the disbelief (kufr) of anyone who embraces a religion other than Islam. Declaring disbelievers, whose disbelief is certain, to be disbelievers (takfir) is obligatory. Not declaring them disbelievers is an act of disbelief itself. However, for someone whose disbelief is not yet established—such as a Muslim who committed an act of disbelief due to error or ignorance—they are not declared a disbeliever until the proof (hujjah) is established against them. As for obsessive doubts (waswas), they must be repelled and ignored, because giving in to them leads to great evil. The fundamental principle is that a Muslim remains on their Islam until it is certainly proven otherwise.