Muir, Marcus Dods and others have suggested that Muhammad was at first sincere, but later became deceptive. Koelle finds "the key to the first period of Muhammad's life in Khadija, his first wife," after whose death he became prey to his "evil passions." Samuel Marinus Zwemer, a Christian missionary, criticised the life of Muhammad by the standards of the Old and New Testaments, by the pagan morality of his Arab compatriots, and last, by the new law which he brought.Zwemer suggests Muhammad defied Arab ethical traditions, and that he personally violated the strict sexual morality of his own moral system. Quoting Johnstone, Zwemer concludes by claiming that his harsh judgment rests on evidence which "comes all from the lips and the pens of his [i.e. Muhammad's] own devoted adherents."Zwemer, "Islam, a Challenge to Faith" (New York, 1907)
