Ruling 207
If a disbeliever says the shahādatayn but one does not know whether he has sincerely become a Muslim or not, he is pure. The same applies if a disbeliever says the shahādatayn and one knows he has not sincerely become a Muslim but he does not do anything that contradicts his saying of the shahādatayn.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)