The doctrine of "monotheism" or Unity of God is the central core of the Abrahamian Religions. This attribute, which from beggining has been a discriminated line between faith and idolatry, is manifested in Judaism, Christianity and Islam. However, In Judaism and Islam, unity of God is a pure unity, while in Christianity we faced with Trinity, i.e the embodiment of three persons (Father, the Son and the Holy Spirit) in one essence (God). According to Bible, the Qur’an, and the words of greatest figures of these religions, this article is going to provide an explanation of the doctrine of divine unity and pure monotheism. At the end, we will criticize the doctrine of Trinity and Unity of God in Christianity.