Malays as defined by the Malaysian Constitution is a curious thing: it is not strictly a race, or an ethnic, but rather an ethno-religious group of people. That simply adds the religious component into the classification of a race that would otherwise be strictly defined and passed down through direct inheritance. According to the Malaysian Constitution, a citizen who is not a muslim is not a malay, even if his lineage traces back to as far and as old as the Malay ethnicity can be traced back to.
But it is not this definition in the Malaysian Constitution that this post will center upon, but rather, this association of religion to a certain ethnic, namely the association of Islam to the Malay ethnic.