I feel like we have just come full circle. I see no reason to assume that the Qur’an is talking about an “orthoodox” doctrine of the Trinity, etc., etc. It is instead reacting to Arabian Christianity as the earliest Islamic community seems to have actually encountered it, etc., etc. And the Arabian Christians with whom early Islam is interacting may not have been Roman Christians, but they seem to talk a lot like Paul did.
That the Arabian Christianity which the Prophet seems to have actually interacted with doesn’t agree with your Christianity is … not an objection to the Qur’an.
The language of the Quran presents us a problem in that regard. It says the Christians, not the Christians of Najran nor a sect of Christians. It uses generic language when the matter is specific. If the source is from God, surely God would have had a knowledge of what the Trinity was by the majority of true Christians. If He is trying to instruct Mohammad and the Muslims on the other two Abrahamic faiths, then surely he would be able to instruct them according to the much larger beliefs within that faith. If the Quran only addresses Arabic Christians of the 7th century (as you yourself say), then it presents Muslims with only a limited and misrepresented understanding of Christianity. And if one has to understand that it only speaks of a limited number of Christians by going to other sources, then the Quran is not self-sufficient.
Let me put things into perspective: suppose a man believes he receives messages from God. He believes he’s in line with Jews, Christians and Muslims. One day he gets a visit by Ahmadi Muslims. The Ahmadi Muslims tell him that Jesus went to India and was buried there. The man suddenly has a revelation condemning this belief, saying in many different ways: “Tell the Muslims that Jesus did not die in India.” That revelation goes into his holy book, which he says comes from God. Now his followers, who are commanded to convert and reach out to Muslims, are left with passages which address only to a small number of Muslims.
If God had wished to convey the beliefs of a certain group, one would think that, in His omnipotence, He could have addressed further beliefs than those of a small group of Muslims who came in and argued with Mohammad one day. Furthermore I would assume that a scriptural understanding of the Trinity would be a bit stronger than one group of Christians “seem to talk a lot like Paul did.”
If you look prior to modern times, distortions abound on both sides, primarily out of ignorance. Distortions that remain, on both sides, usually still have the same source (I mean, how many times I am going to have to hear that Muslims don’t believe God is Love from, often, well educated Christians…).
That there exist ignorant Christians is not undeniable, but that does not forgive ignorance from another side - that’s a
non sequitor. That doesn’t answer why every Muslim except for yourself seems to believe that the Quran addresses the belief of all Christians.
To address your specific question, I don’t think many Muslims take the time to really understand the Christian account of the Trinity. Unless you are really really interested in Christianity for some reason (and then, almost by definition, you would have to be a Muslim who is deeply rooted in the “West” somehow, for whom Christianity is a living, daily, cultural reality with which you are confronted in a bewildering myriad of forms) there is little motivation to learn the intricacies of a confusing Christian idea, that most Christians don’t themselves understand, and that even after you make all the effort still turns out to be fundamentally wrong. When you take up the Conciliar Trinitarian definition and actually try to talk about it, for instance, the sample size of people you are talking to is exceedingly small (despite the fact that it is supposedly normative for most Christians).
As I said to Hamba earlier in this thread, the consistency in Trinitarian teaching is much more consistent than most Muslims here seem to give it credit for. If I were to put St. Ignatius, St. John Chrysostom, St. John Damascene, St. Photios, Martin Luther, John MacArthur and James White together in a room and asked them to define the Trinity, they would all define it the same way.
The existence of individual Christians who do not understand the doctrines of their faith means nothing. There exist Muslims who do not understand their own faith, along with Jews, Hindus, Buddhists and others. That does not denote that a doctrine has a definition.
If I were telling a local masjid why the Trinity is wrong, what would I tell them? I would tell them that despite the fact that Christians claim to worship the One, they introduce multiplicity into God. That is all your average Muslim really needs to know in order to talk to their average Christian next door neighbor and understand the difference that is present between them.
That is still wrong. Multiplicity implies numerous and many. That’s not what the Trinity is. Each Person within the Trinity represents the Fullness of God, not three separate Beings. You would therefore be teaching your Muslims the incorrect definition of the Trinity.