How did early Islam gain its understanding of Christians and Christianity? How did it react to Christian claims of universality? To answer these questions, this article first identifies passages pertaining to the Christian religion in representative texts of the three main bodies of literature produced by the first Muslim writers and editors: the Qur'ān, the Hadith and the Sira. This data is then analyzed into five ideal-types of Islamic attitudes toward Christians and Christianity: 1) affirmations of the truth of the Gospel, 2) descriptions of Christians as true believers, 3) descriptions of Christians as sectarians, 4) accusations of disbelief (kufr) and 5) accusations of idolatry (shirk). The assertion of an historical relationship between sectarianism, disbelief and idolatry led to subordinating the Muslim-Christian dialogue on the recognition of the unicity of God, which could be conceived of as providing the basis either for a restricted religious pluralism or for an Islamic universalism.