Monotheism is the belief in one God. It is the essence of Judaism.
The Jewish belief in one God can be found in the first two of the Ten Commandments. The first, “I am the Lord your God who brought you out of Egypt, the house of bondage” (Exodus 20:2), reaffirms the truth of God’s being.
The second, “Thou shalt have no other gods before me” (Exodus 20:3), is the negative complement to the first commandment and a renunciation of idolatry.
It is an understood principle in Judaism that idolatry does not solely constitute atheism, a lack of belief in God. Idolatry also does not translate to replacing the Jewish God with another Higher Power. (Though both of these ideas are considered components of idolatry.)
Idolatry means denying God’s oneness. If one believes God has partners in God’s work or compartmentalizes God, this is idolatry.
Some Jews believe Christianity to be idolatry for this reason, though the Jewish community is deeply divided on this issue. Some Christians prescribe literal Godliness to the “Father, Son and Holy Ghost,” which goes against the Jewish idea of monotheism, again the essence of Judaism. Islam is considered a monotheistic religion by all Jews. Allah, the Muslim word for God, is thought to be synonymous with HaShem.