How much admissible and valid of a Christians belief of God who believes in trinity?

Belief should be considered as a whole. One can be called as a believer if only he/she believes all six pillars of belief. Anybody who has no faith even one of these principles or if his/her belief would not be compatible with Islam, he/she could not be considered as a believer according to Quran. Unification (Tawheed) is the principle cause of the Quran. It means God is One; He has no need of any partner or a peer for his essence, attributes, and actions. Moreover, any belief, which contradicts with Tawheed (Unification), is Shirk (polytheism). This kind of belief is not acceptable before Allah.

As already known, there are three kinds of religions: Samawi (monotheistic) religions, distorted religions, and false beliefs (superstition). The Religion before Allah is Islam (Al-Imran Surah, 3:19) with the unmistakable decree of the ayat (Quranic verse), just like humankinds product of false beliefs, Judaism and Christianity have lost of their being Samawi (monotheist) by the distortions which had been made to their holy books.

If anyone desires a religion other than Islam, never will it be accepted of him; and in the Hereafter he will be in the ranks of those who have lost. (Al-Imran Surah, 3:85)

When it is said Religion, one remembers a firm belief and a worship. According to this; Belief in God, Angels, Holy books, Prophethood, Akhirat (life after death), and Khadar (predestination and decree) in other belief systems are not compatible with the truth.
If one is standing, it is because of its equipment, rule is a well-known one. There are unavoidable requisites, special occasions, and conditions for anything to happen. You cannot think of this particular thing with the absence of these things. For example when the concern is spirit, life is requisite of it. You cannot think of life without a spirit. Another important firm belief rule is Belief does not accept disassociation. That is to say, one cannot think of principles of belief separately and have no right to choose of those.

For example, one who believes in Allah but not in Akhirat is not considered as a believer. It cannot be said that believer of Allah but unbeliever of Akhirat. Belief in Allah does not accept any disassociations.

A person who says, I believe in Allah but do not accept His being Qadeem (original and absolute) actually believes to a deity which he fantasizes in his mind.
According to these two rules, for believing in Allah to be true all six-principle causes of belief have to be believed as Quran states. Because it is the perfect book which introduces Allah to humans and jhins; and it is the only Samawi Book, which had not been distorted.
As already known, the attributes of Allah are divided into two as Salbiyya (Personal) attributes and Subuti (Affirmative) attributes. Personal attributes of Allah are Wujud (existence), Qidam (eternity), Baqa (everlastingness), Mukhalafatu lil-Hawadith (non-resemblance of the creation), Qiyam bin Nefees (non-neediness of others), Wahdaniyyah (oneness). Existing with a necessity being, being eternal and everlasting, not resembling any other, existing because of Himself and not being in need of any peer, and being One cannot be ascribable to any other deity other than Allah.

Affirmative attributes of Allah (swt) on the other hand are Hayah (life), Ilim (knowledge), Iradah (will), Qudrah (power), Sam (hearing), Basar (sight), Kalam (speech), Taqwin (creating). The only One who has all seven of these attributes is Allah. When we recite La Ilahe Illallah, we speak all of these attributes.

When it is said Belief in Allah, it means belief in all of these attributes. When one is missing, it cannot be called as belief in terms of Quran.

When we revise Quran, we see that that are given many different messages prior to and just after the decree; There is no god but He. Let us feature some of those:

They do blaspheme who say: God is one of three in a trinity: for there is no god but One God (Allah) (Al-Maidah Surah, 5:73). Therefore, they who believe in trinity negate and estrange from the truth.

Say: O men! I am sent to you all, as the Messenger Allah, to whom belongs the dominion of the heavens and earth: there is no god but He: It is He that gives both life and death. (Al- Araf Surah, 7:158) Therefore, one who believes to a deity who is not the owner of heavens and earth, one that is mortal and in need of help from Allah to resurrect in the judgment day or one that ascribes a partner to Allah cannot be considered as a true believer of Allah.

Allah! There is no god but He: of a surety He will gather you together against the Day of Judgment about which there is no doubt. (Al-Nisaa Surah, 4:87) It cannot be called as a deity if he is unable to gather people on Judgment Day. He it is who shapes you in the wombs as he pleases. There is no god but He, the Exalted in Might, the Wise. (Al-Imran Surah, 3:6) It cannot be called as a deity for the one whom had been shaped in the womb by Allahs Will.

Is there a creator other than Allah to give you sustenance from heavens or earth? (Fatr Surah, 35:3) God is One who provides our livelihood by running Heavens-Earth together in proper order just like a factory.

Say: He is my Lord! There is no god but He! On Him is my trust, and to Him do I turn! (Ar-Rad Surah, 13:30) To hope help from humans, supposing that they can forgive ones sins cannot be considered as a belief according to Quran.

Another verse about Tawheed: He is the First and the Last, the Evident and the Immanent: and He has full knowledge of all things (Al-Hadid Surah, 57:3). For one who has a beginning and an end, who is under the authority of Allah cannot be called as a deity.
There are so many lessons in these verses, which are from Quran for those who believe in trinity. Hazrath Jesus (pbuh), before being anything is a human being. But not an ordinary one. He was honored with Risalet (prophethood). His mother was also honored of being the mother of a prophet. It is so hard to claim that they have faith in Allah for those who go to far by referring them as deities in terms of Quran.Author:
Alaaddin Başar (Prof.Dr.)