Some Sufis claim to be Muslim while they are polytheists.
Will anyone who does not condemn them as disbelievers also be considered a disbeliever? What is the evidence for this?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

A Muslim must be careful and not hasty in calling others disbelievers. This is a very serious matter, and its consequences include marital separation, deprivation of all rights of inheritance and deserving to be doomed in Hell, if a person is indeed established as a disbeliever.

Therefore, there is a strict prohibition on labeling Muslims with disbelief unless certain conditions are fulfilled and there are no impediments preventing one from applying such ruling to them. Ibn Taymiyyah said that no one is permitted to condemn a Muslim as a disbeliever, even if he has committed mistakes, until he is educated beyond the excuse of ignorance and doubt and is shown the truth. A person cannot condemn a Muslim as a disbeliever merely based on doubts. Rather, he is only to be considered a disbeliever after having been educated beyond the excuse of ignorance and having had his doubts dispelled. 

Allaah Knows best.