Within Islam, however, such a concept of plurality within God is a denial of monotheism and foreign to the revelation found in Muslim scripture. Shirk, the act of ascribing partners to God – whether they be sons, daughters, or other partners – is considered to be a form of unbelief in Islam. More details about Shirk in article “Shirk, i.e. placing a person or ideology at God’s level.”

O People of the Book! Commit no excesses in your religion: Nor say of Allah aught but the truth. Christ Jesus the son of Mary was (no more than) an apostle of Allah, and His Word, which He bestowed on Mary, and a spirit proceeding from Him: so believe in Allah and His apostles. Say not "Trinity" : desist: it will be better for you: for Allah is one Allah. Glory be to Him: (far exalted is He) above having a son. To Him belong all things in the heavens and on earth. And enough is Allah as a Disposer of affairs. (Quran 4:171)

In religion people's excesses may lead them to blasphemy or a spirit the very opposite of religion. The Jewish excesses in the direction of formalism, racialism, exclusiveness, and rejection of Christ Jesus have been denounced in many places. Here the Christian attitude is condemned, which raises Jesus to an equality with Allah: in some cases venerates Mary almost to idolatry: attributes a physical son to Allah: and invents the doctrine of the Trinity, opposed to all reason, which according to the Athanasian Creed, unless a man believes, he is doomed to hell for ever. Let our Muslims also beware lest they fall into excesses either in doctrine or in formalism. The actual problem of the Islamic Umma comes from this point. Islam is the most tolerant and most recipient religion while some of our Muslims are the most intolerant and most segregating.

They do blaspheme who say: Allah is one of three in a Trinity: for there is no god except One Allah. If they desist not from their word (of blasphemy), verily a grievous penalty will befall the blasphemers among them. (Quran 5:73)

Similar content in 5:116-117.

The first manifestation of the Holy Spirit in the Bible is when Abraham hosts the three angels. In the Quran, this theorem is confirmed in 11:77, in the interpretation of Qurtubi they are Gabriel, Michael, and Israfil. Its first mention in the Bible was when the angel Gabriel brought the “good news” that by the Holy Spirit Mary would be the mother of the Messiah. In Islam Gabriel is meant under the concept of the Holy Spirit. In the Quran, the angels’ visit to Mary is recorded in 3:45-47. Islam does not unite Gabriel with the Father and Son into one doctrine. (See article: “Gabriel (Jibreel) and the angels”). The emergence of the doctrine of Trinity is considered by Islam as a consequence of a controversy between Presbyter Arius (256-336) and Bishop Athanasius (296-373). A two-month synod began in May 325 in Nicaea, Asia Minor. Arius and his followers believed that Jesus was the most perfect creature, but a human being, so he could not be equal to the divine power that created him. Patriarch Athanasius and Alexandros rejected the human nature of Jesus and, like the Father, considered him eternal with divine greatness. Arius denied the doctrine of Trinity but Athanasius professed it. The Council of Nicaea ended up with the victory of Athanasius, and from then the doctrine of the Trinity and the quality of Jesus was codified accordingly in the Catholic Church. Arius' teachings were declared as heretical, yet those laid down the foundation of the Unitarian religion. Jesus promised in the Bible the coming of the Holy Spirit, which is called “Paracletos” in the Greek Gospels and is often translated as “Comforter”.

The same in the Quran as follows:

And remember, Jesus, the son of Mary, said: "O Children of Israel! I am the apostle of Allah (sent) to you, confirming the Law (which came) before me, and giving Glad Tidings of an Messenger to come after me, whose name shall be Ahmad." But when he came to them with Clear Signs, they said, "this is evident sorcery!"

The mission of Jesus was to his own people, the Jews. Cf. Matt. 10:5-6. See also Matt. 15:24: "I am not sent but to the lost sheep of Israel;" also Matt. 15:26: "It is not meet to take the children's bread, and to cast it to dogs."

"Ahmad", or "Muhammad", the Praised One, is almost a translation of the Greek word Periclytos. In the present Gospel of John. 14:16, 15:26, and 16:7, the word "Comforter" in the English version is for the Greek word "Paracletos", which means "Advocate", "one called to the help of another, a kind friend", rather than "Comforter". Our doctors contend that Paracleots is a corrupt reading for Periclytos, and that in their original saying of Jesus there was a prophecy of our holy Prophet Ahmad by name. Even if we read Paraclete, it would apply to the holy Prophet, who is "a Mercy for all creatures" (21:107) and "most kind and merciful to the Believers" (9:128).

In summary, Islam considers Jesus (peace be upon him) to be a man, prophet, servant of God, His soul and word whom He (SWT) has cast into Mary. In the interpretation of Islam, he is a Muslim, a prophet submitted to the One God. The doctrine of father, son, holy spirit is rejected by Islam. It considers that as a human creation. Islam is closer to the Unitarian teachings of Arius in this regard. The events that Christianity attributes to the Holy Spirit, Islam attributes to angel Gabriel. In the present Gospel of John. 14:16, 15:26, and 16:7, the word "Comforter" according to some of our interpreters is reference to prophet Mohammed (peace be upon him).

Behold, our doctrines are differing here, but at the same time we must respect each other. When the stake is whether the forces of corruption will reign on our earth, the point is not the origin of our prophets’ roots, but their teachings. Regarding our teachings we are close to each other. We may act more efficiently if we knew each other better and concentrate on points which are common.