Islam embraces one God (monotheism) but not one religious tradition. Muhammad’s visions showed that all prophets of God including Moses, Jesus, Ishmael, Isaac, and Jacob are valid as are their teachings. In fact, Karen points out that believing in one faith could lead to degeneration placing belief in the tradition of that faith over belief in God.
An infidel, or unbeliever, is commonly understood by Westerners to be someone who is not a Muslim. An infidel (kafir in Arabic language) is more accurately defined as one who does not believe in the one God. That does not include Christians, Jews and others who are known instead as the “People of the Book” in the Qur’an. An infidel refers more to atheists and polytheists. One could not be a Muslim without believing in Jesus.
The common Western understanding of an infidel is most likely an understanding that is portrayed by some political factions associated with Muslims to advance their own agenda; an agenda that is not consistent with Islamic principles and exploits general Western ignorance of Islam.