Christians DEVIATED from Original Teachings of Isa(Jesus), the Messenger of ALLAH.
The Muslims are taught that PRIOR to the advent of Muhammad, the Final Messenger of Allah, and the religion of Islam, there were earlier Messengers of ALLAH being sent to mankind, all with the same basic message, WORSHIP ALLAH and DO NOT ASSOCIATE OTHERS with Him.
The Quran mentioned at least 25 Messengers of ALLAH, including Adam, Noh(Noah), Yaakub(Jacob), Yusuf(Joseph), Daud(David), Sulaiman(Solomon), Ibrahim(Abraham), Ismail(Ishmael), Ishak(Isaac), Musa(Moses), Isa(Jesus) and Muhammad(Mohamad). They were sent at different period of time, to different tribes of mankind.
All above-mentioned Messengers of ALLAH were selected by ALLAH, to convey to manking TRUTHS about the Universe, including the knowledge of ALLAH as the Creator and Sustainer of the Universe, and ALLAH as the only God to be worshipped. And those who followed them, worshipped none other than ALLAH, even before the advent of Muhammad, the final Messenger of ALLAH.
During the lifetime of a particular Messenger of Allah, his followers would be following his teachings strictly. But, upon their demise, the followers tend to go back to their old ‘bad habit’ of worshipping ‘idols’ and FALSE GODS, or modify the teachings of the Messenger. When this happened, ALLAH would send another Messenger to bring back people to the right path.
Indeed, it was the same ALLAH who had appointed Isa(Jesus) as a MESSENGER OF ALLAH, to bring back the LOST SHEEP OF ISRAEL, who deviated from the earlier teachings of Musa(Moses), an earlier Messenger of Allah. However, after the elevation of Isa(Jesus) temporarily to heaven, as revealed to Muslims(as against the Christians belief that Jesus was crucified), the later Christians had OVER VENERATED Isa(Jesus), transforming him from a PERFECTLY NORMAL HUMAN BEING to be SON OF GOD. Worse still, the SON OF GOD was made a COMPONENT of TRINITY GOD, beside the other component, the HOLY SPIRIT.
By doing that, the Christians had DEPARTED from the most fundamental teachings of earlier Messengers of ALLAH, to worship ALLAH alone, and not to bring any other element to be ASSOCIATES with the one and only God, ALLAH. Chriatians’ FABRICATION of Jesus and ‘Holy Spirit’ as the SECOND and THIRD components of God was/is their BIGGEST LIE to mankind. It made a MOCKERY of one and only true God, ALLAH. It lowers the GREATNESS of ALLAH, to a level fit to father a human being by the name of Isa(Jesus).
Muhammad, the Final Messenger of ALLAH was sent to put back mankind on the right path after the DEVIATION of the Christians from the original teachings of Isa(Jesus), the Messenger of ALLAH.
This is the fundamental difference between Christians understanding of God, and the Muslims understanding of God.
The Muslims do not want the one and only true God, ALLAH, be adopted as the name of God of the Christians, because the Christians have DEGRADED the only one true God, ALLAH, to the level of human being, by fathering the HUMAN BEING Isa(Jesus).
The Muslims are taught that ALLAH is the one and only God for all the Universe, and all within the Universe, including all human beings, Muslims and Christians included. But the muslims do not want the term ALLAH be used as God in the Christian sense, after the Christians had DEGRADED the GREATNESS of ALLAH, to the level of human.