Monotheism is the belief that there is one God who created the Universe in all of its vastness and intricacies, created human life to be valuable compared to other creatures, set the Laws of Nature in place, and now oversees the world. This God is omnipresent (present everywhere), omniscient (all-knowing), and omnipotent (all-powerful). There are three main monotheistic worldviews, including Judaism, Christianity, and Islam.
Judaism is a religion that was established by Ancient Hebrews, who believed the Creator of the Universe revealed himself to the Prophets, such as Abraham, Isaac, and Jacob. Judaism uses the Torah of Truth and the New Testament of the Bible, as history from the beginning of the world to the time of the last of the prophets and the law to which they need to live to earn eternal life. Judaism uses these historical documents, revelations from God, and science to support the order of the universe, as their way of knowing Truth. Since Judaism is an ethnic faith, they do not have missionaries, but they believe the Gentiles (not Jewish people) can achieve salvation by living according to the Ten Commandments.
Christianity is very similar to Judaism in that it seeks truth from historical documents (including the Old and New Testament), uses science to support an ordered universe, and uses revelations from God to seek Truth. There are, of course, many differences-- the largest being that Christians believe that Jesus fulfilled the prophecy of a promised Messiah in the Old Testament; modern Judaism rejects Jesus as Savior and are still waiting for one. The Old Testament promises a Savior who overthrows the authority, takes back Israel, and builds a New Temple. Jesus was born humbly in a manger, overcame the world by defeating death on a cross, and said the body was now the third temple because he came as a mediator between God and man, so that all people may have a personal relationship with God. He fulfilled the law, and Salvation is achieved by asking him to be Savior of one's life and then living according to his will: "The gospel is the power of God unto salvation to everyone who believes, to the Jew first and also to the Gentille" (Romans 1:16). Christians also believe that the New Testament is Truth (the Old Testament beginning with creation of Earth and the New Testament with the end of Earth). However, they do not believe any other documents should be made equal to the Bible until the return of Jesus during the end of the world. Christians believe in the Holy Trinity that God the Father, Jesus Christ the Son, and the Holy Spirit make up the one Almighty God. Anyone who accepts Jesus as Lord of their life and listens to his teachings will receive the Holy Spirit, who will then reveal Truth to them. As Jesus said, "Then you will know the truth, and the truth will set you free" (John 8:32).
Islam is the belief in Truth through the revelation of the Prophet Muhammad, who brought the Quran. Although Muslims do not exactly dismiss the Bible, they believe the Testaments are inferior to any contradictions to the Quran (of which there are many). The Quran was seen as an instruction to build the true church of Allah, after Christians and Jews had failed to deliver the correct message. In Islam, there are five main pillars that should be followed to achieve eternal life, rather than by the Grace described in the Christian faith. In Islam, a personal relationship with God is seen as blasphemy to his power. Jesus is considered to be a prophet, but not an equal to God. The Holy Spirit is believed to be the Angel Gabriel, who revealed the Quran to Muhammad.