Q. I accept that it is not permissible to perform Namaaz with a mask. But why will the Namaaz not be valid?
A. The Namaaz will not be valid because the donning of the mask comes with the kufr adwaa (contagion) belief. This kufr theory of the atheists effaces Imaan. Thus, Namaaz will not be valid. In fact, the one who wears the niqaab of Iblees believing in the kufr theory, should renew his Imaan.