A respectful examination
The Trinity is the dividing line between Christianity and the faith of Jesus as Muslims understand it. Here is what the Bible, history and reason say about it.
Most Christians have never been shown a single verse in which Jesus says he is God or describes God as three persons. That is not an accident of memory - no such verse exists. What exists is a doctrine built over three and a half centuries, out of philosophical terms (ousia, hypostasis, homoousios) that appear nowhere in Scripture. Below, point by point, is why Muslims - and many sincere Christians through history - have concluded that the Trinity is not what Jesus taught.
Neither Jesus nor any prophet ever used the word 'Trinity' or taught that God is 'three persons in one essence'. The term (Latin trinitas) was first used by Tertullian around 200 CE - almost two centuries after Jesus. The doctrine was formalised at the Council of Nicaea (325 CE) and the Council of Constantinople (381 CE), by vote, under the Roman emperor. The Holy Spirit was not declared fully God until 381 CE.
1 John 5:7 ('the Father, the Word, and the Holy Ghost: and these three are one') does not exist in any Greek manuscript before the 14th century. Scholars call it the 'Comma Johanneum'. The NIV, ESV, NRSV, NASB and almost all modern Bibles omit it or footnote it as not original. If the Trinity were central to the faith of Jesus, it would not depend on an interpolated line.
For there are three that testify: the Spirit, the water and the blood; and the three are in agreement.
If God were a Trinity, this was the moment to say so. Instead, Jesus quoted Deuteronomy 6:4 word for word, and the Jewish scribe - who certainly understood it as strict monotheism - was told he was 'not far from the kingdom of God' (Mark 12:34).
The first of all the commandments is, Hear, O Israel; The Lord our God is one Lord.
He prayed to God (Luke 6:12), was sent by God (John 17:3), received commandments from God (John 12:49), did not know what God knows (Mark 13:32), could do nothing of himself (John 5:30), called God 'my God' (John 20:17), and said 'my Father is greater than I' (John 14:28). An all-powerful, all-knowing God does not pray, learn, grow, hunger, sleep or die.
Every other core teaching - love of God, love of neighbour, forgiveness, prayer, charity - Jesus stated plainly and repeatedly. The most important fact in the universe, if true, that God had become a man, is never stated by him in a single clear sentence. Christians are left with inferences from 'I and my Father are one' (which he also prayed for his disciples) and 'before Abraham was, I am' (a phrase the healed blind man also used of himself, John 9:9).
Peter preached Jesus as 'a man approved of God' (Acts 2:22) and 'his servant' (Acts 3:13, 3:26 - Greek pais). Paul wrote: 'there is one God, and one mediator between God and men, the man Christ Jesus' (1 Timothy 2:5) and 'the head of Christ is God' (1 Corinthians 11:3). Early Jewish-Christian groups such as the Ebionites and Nazarenes, who followed James the brother of Jesus, held Jesus to be a human prophet and Messiah.
But to us there is but one God, the Father, of whom are all things, and we in him; and one Lord Jesus Christ, by whom are all things, and we by him.
Adam, Israel, David, Solomon and the peacemakers are all called sons of God in the Bible (see the Bible page). 'Lord' (kyrios) was the ordinary Greek term for 'sir' or 'master' - Sarah called Abraham 'lord' (1 Peter 3:6). The word 'worship' (proskuneo) in the Gospels means to bow down and was offered to kings and prophets as well (1 Chronicles 29:20, where the people 'worshipped the Lord, and the king').
'God is not a man' (Numbers 23:19). 'I am God, and not man' (Hosea 11:9). God 'cannot be tempted with evil' (James 1:13), yet Jesus 'was in all points tempted' (Hebrews 4:15). God does not die (Habakkuk 1:12), does not change (Malachi 3:6), is not seen by any man (John 1:18). The Trinity requires saying Jesus was 'fully God' while lacking God's knowledge, power, immortality and invisibility - a formulation the Church itself calls a 'mystery'.
God is not a man, that he should lie; neither the son of man, that he should repent.
The Quran does not ask Christians to think less of Jesus. It asks them to return to what he taught: 'Indeed, Allāh is my Lord and your Lord, so worship Him' (Quran 3:51; compare John 20:17). It warns against 'excess' in religion - raising a servant of God to be God Himself.
O People of the Scripture, do not commit excess in your religion or say about God except the truth. The Messiah, Jesus, the son of Mary, was but a messenger of God and His word which He directed to Mary and a soul from Him. So believe in God and His messengers. And do not say, ‘Three’; desist - it is better for you. Indeed, God is but one God. Exalted is He above having a son.
c. 30 CE
Jesus teaches 'The Lord our God is one Lord' (Mark 12:29) and is raised to God.
c. 50-100 CE
Gospels and letters written in Greek, decades after Jesus, by authors who never claim he said 'I am God'.
c. 200 CE
Tertullian coins the Latin word 'trinitas'. Many church leaders still hold the Father alone to be God in the full sense.
325 CE
Council of Nicaea, convened by the pagan-raised Emperor Constantine, votes that the Son is 'of the same substance' as the Father. Arius and those holding Jesus to be a created being are exiled.
381 CE
Council of Constantinople declares the Holy Spirit also to be God - completing the Trinity 350 years after Jesus.
451 CE
Council of Chalcedon defines Jesus as 'fully God and fully man', splitting churches that still disagree today.
610 CE
The Quran is revealed, calling the People of the Book back to the original message: 'our God and your God is one' (29:46).
God is One. Jesus is His Messiah and messenger. The Holy Spirit is His angel Gabriel, who brought the revelation. This requires no “mystery”, no councils, no philosophical vocabulary. It is what a child can understand, what Abraham and Moses taught, what Jesus answered when asked, and what the Quran restored.
And this is life eternal, that they might know thee the only true God, and Jesus Christ, whom thou hast sent.
The Messiah, son of Mary, was not but a messenger; [other] messengers have passed on before him. And his mother was a supporter of truth. They both used to eat food. Look how We make clear to them the signs; then look how they are deluded.
Matthew 4:10. That is Islam: submitting to the One God alone, exactly as Jesus did. Take a moment to see how simple the next step is.