History of Christianity and Shift from Monotheism to Trinity

Controversy rages between Christians and non-Christians, including Muslims and the followers of the other faiths, and among Christians themselves over the issues of monotheism and the Trinity.
Though the Trinity has become a prevalent Christian doctrine recently and the monotheists have become a minority compared to the other Christian denominations, history tells us that the doctrine of the Trinity was not the prevalent Christian doctrine in the past, but it was rather such a doctrine which found favor with the idolatrous Roman Empire which newly converted to Christianity.
Pure monotheism had been and remained the very basis of the Christian creed until it was overshadowed by the doctrine of the Trinity which was supported by the idolatrous authority. However, monotheism continued to exist and be professed by considerable fellowship until Prophet Muhammad came up with the final monotheistic message of Islam and asserted monotheism as the doctrine explicitly preached by Jesus, and disowned all forms of Trinity, polytheism, and paganism which were introduced into the genuine Christian faith.
Therefore, monotheistic Christians embraced Islam for they found out that it is exactly identical to the previous divine messages of both Moses and Jesus as well as the revelations sent down to them.

The Qur’an often harmonizes with the Bible, especially the New Testament, over the reported statements and acts of Jesus, as well as the incidents which took place, the quotations which were cited and the controversy which broke out during the period of time Jesus was sent.

The Qur’an tells us that the disciples believed in God as the Deity, as well as in the revelation He sent down and in Jesus as the prophet and messenger of God. For example, God says:

But when Jesus felt [persistence in] disbelief from them, he said, “Who are my supporters for [the cause of] God?” The disciples said,” We are supporters for God. We have believed in God and testify that we are Muslims [submitting to Him]. Our Lord, we have believed in what You revealed and have followed the messenger [Jesus], so register us among the witnesses [to truth].”(Aal `Imran3:52-53)

God also says:

And [remember] when I inspired to the disciples, “Believe in Me and in My messenger [Jesus].” They said, “We have believed, so bear witness that indeed we are Muslims [in submission to God].” (Al-Ma’idah 5:111)

The verses of the New Testament concur with those of the Qur’an in this regard. Jesus is unequivocally described as “God’s messenger” in the New Testament. We read: “And so, dear brothers and sisters who belong to God and are partners with those called to heaven, think carefully about this Jesus whom we declare to be God’s messenger and High Priest. He was faithful to the one who appointed him, just as Moses was faithful in all God’s house.” (Hebrews 3:1-2)