Translating Muslim as credere undercut Islam’s claim that all prophets followed it.

Evidence

In doing this those who are learned in religion as well as the uneducated follow the good religion.’ In the passage a little later about the religion of Abraham, credere was again used, although the argument that Abraham preceded both Jews and Christians was presented clearly enough. … This was deliberately to ignore the claim of Islam to be and to have been the religion of all the Prophets, while accepting that Muhammad pretended that his prophethood belonged in the grand tradition. — Norman Daniel, Islam and the West, 153