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Monotheism in Islam 

After the Ascension of Prophet Jesus (pbuh) to heaven, most of the Christians remained Unitarian. They used to believe in the Oneness of Allah following the teachings of Prophet Jesus (pbuh) and all the other Prophets. They did not believe in the doctrines of Trinity, Original Sin, Blood Atonement or Divine Sonship of Prophet Jesus (pbuh). After centuries of persecution against the Unitarian Christians, the Pauline Church prevailed and the teachings of Paul replaced the teachings of Prophet Jesus (pbuh) and Paul became the real founder of today's Christianity.

Paul, the self-appointed thirteenth apostle who never saw Jesus (pbuh), wrote more Books of the Bible than any other author, while Prophet Jesus (pbuh) did not write a single word, as the original Gospel revealed by Allah to Jesus (pbuh) was lost.

However, Paul replaced the Gospel of Jesus (pbuh) with a gospel written by him and invented the doctrines of Original Sin, Blood Atonement and Divine Sonship of Prophet Jesus (pbuh). With the exception of few verses, all the verses in Paul's letters are his own words and not the words of Prophet Jesus (pbuh):

Remember that Jesus Christ of the seed of David was raised from the dead, according to my (Paul) gospel. (2 Timothy 2:8} (KJV)

After the loss of the original Gospel of Prophet Jesus (pbuh), many gospels were written. These gospels include some of the genuine teachings of Prophet Jesus (pbuh) mixed with people's own texts and interpretations which were influenced by the beliefs of that period. Therefore, it became difficult and sometimes impossible to know which parts came from Allah and which from people. As a result, Christians deviated from the teachings of Prophet Jesus (pbuh) and followed the teachings of hundreds of monks and priests which resulted in hundreds of religious groups having different beliefs.

You will find articles in this segment that discuss these four topics in more details.