I have engaged recently in an internet dialog on the subject of whether Mohamed, the alleged prophet followed by Islam, understood the Trinity. I conclude that the answer is "no," and I have a demonstration prepared.
1. Surah 5 is Mohamed's Writing
Surah 5 is part of the Koran, and consequently one of the writings of Mohamed (though not necessarily penned by him personally). Because the early manuscripts of the Koran are limited, we will be dealing with a translation of the Koran into English based on the dominant family of Koranic texts, the Uthmanic rescension. For the purpose of this analysis, we will presume that Uthman made the correct textual critical choices, and - as well - that the English translation is reasonably accurate. There are certainly other English translation available. The English translation I am providing can be found here (link).
2. Surah 5 addresses the Jews and Christians generally
There are a number of issues addressed in Surah 5. In the first portion of the Surah, various dietary laws are stressed, main similar to Old Testament Jewish laws and the New Testament Council of Jersalem restrictions.
There is what seems at first to be an aside in verse 3:
"This day have those who disbelieve despaired of your religion, so fear them not, and fear Me. This day have I perfected for you your religion and completed My favor on you and chosen for you Islam as a religion; but whoever is compelled by hunger, not inclining willfully to sin, then surely Allah is Forgiving, Merciful."
You will notice, however, that "those who disbelieve despaired of your religion" is a reference to the Jews and Christians that Mohamed views as the forerunners of Islam (the "perfected ... religion").
After a few more rules on cleanliness, Mohamed proceeds to some general remaks on good behavior (vs 8): "O you who believe! Be upright for Allah, bearers of witness with justice, and let not hatred of a people incite you not to act equitably; act equitably, that is nearer to piety, and he careful of (your duty to) Allah; surely Allah is Aware of what you do."
And Mohamed attaches a promise to the good behavior, which we should understand to include not only the general goodness but also maintenance of the cleanliness laws (vs. 9): "Allah has promised to those who believe and do good deeds (that) they shall have forgiveness and a mighty reward."
It is plain works salvation, contrary to the teachings of the Bible (both of the Old, but more especially of the New, Testaments).
Then in the next verse, Mohamed cautions against the "disobedient" paths (vs. 10): "And (as for) those who disbelieve and reject our communications, these are the companions of the name."
In verses 11-14, Mohamed suggests that both the Jews and the Christians forsook Islam. He recounts the supposed covenant-breaking of both the Jews and the Christians:
"O you who believe! remember Allah's favor on you when a people had determined to stretch forth their hands towards you, but He withheld their hands from you, and be careful of (your duty to) Allah; and on Allah let the believers rely. And certainly Allah made a covenant with the children of Israel, and We raised up among them twelve chieftains; and Allah said: Surely I am with you; if you keep up prayer and pay the poor-rate and believe in My apostles and asslst them and offer to Allah a goodly gift, I will most certainly cover your evil deeds, and I will most certainly cause you to enter into gardens beneath which rivers flow, but whoever disbelieves from among you after that, he indeed shall lose the right way. But on account of their breaking their covenant We cursed them and made their hearts hard; they altered the words from their places and they neglected a portion of what they were reminded of; and you shall always discover treachery in them excepting a few of them; so pardon them and turn away; surely Allah loves those who do good (to others). And with those who say, We are Christians, We made a covenant, but they neglected a portion of what they were reminded of, therefore We excited among them enmity and hatred to the day of resurrection; and Allah will inform them of what they did."
After this condemnation of both Jews and Christians, Mohamed continues by grouping them together as the "people of the book." He makes a proselytory message to them (I'd call it evangelistic if he were offering the gospel) (vss. 15-16): O followers of the Book! indeed Our Apostle has come to you making clear to you much of what you concealed of the Book and passing over much; indeed, there has come to you light and a clear Book from Allah; With it Allah guides him who will follow His pleasure into the ways of safety and brings them out of utter darkness into light by His will and guides them to the right path."
The point of the message that Mohamed is giving is simple: I have the knowledge that was concealed, therefore you need me.
Next, Mohamed aims a dart at the Christians, arguing in verse 17: "Certainly they disbelieve who say: Surely, Allah-- He is the Messiah, son of Marium [i.e. Mary]. Say: Who then could control anything as against Allah when He wished to destroy the Messiah son of Marium and his mother and all those on the earth? And Allah's is the kingdom of the heavens and the earth and what is between them; He creates what He pleases; and Allah has power over all things."
In other words, the basic argument seems to be this: if Jesus is God, how could he have been destroyed and his mother? The answer, of course, is that Jesus gave his life freely, no one took it from him. He gave it as a sacrifice for the sins of his people.
Mohamed then turns to a joint critique of the Jews and Christians (vss. 18-19(: "And the Jews and the Christians say: We are the sons of Allah and His beloved ones. Say: Why does He then chastise you for your faults? Nay, you are mortals from among those whom He has created, He forgives whom He pleases and chastises whom He pleases; and Allah's is the kingdom of the heavens and the earth and what is between them, and to Him is the eventual coming. O followers of the Book! indeed Our Apostle has come to you explaining to you after a cessation of the (mission of the) apostles, lest you say: There came not to us a giver of good news or a warner, so indeed there has come to you a giver of good news and a warner; and Allah has power over all things."
Here is the next point: Mohamed points to judgment on Jews and Christians as proof that they do not follow Allah. However, (a) God chastens us (Christians) and chastened them (Jews) because of our parent/child relationship to God, and (b) it is not as though judgments do not come on Muslims. The 2004 Indian Ocean tsunami killed more than 200,000 people and left homeless more than a million - largely Muslims.
In verses 22-35, Mohamed essentially makes various arguments from Moses and warns against dibelief. Next, at verses 36-37, Mohamed provides a warning to disbeleivers (referring, essentially, to the Jews and Christians.
"Surely (as for) those who disbelieve, even if they had what is in the earth, all of it, and the like of it with it, that they might ransom themselves with it from the punishment of the day of resurrection, it shall not be accepted from them, and they shall have a painful punishment. They would desire to go forth from the fire, and they shall not go forth from it, and they shall have a lasting punishment."
Then, Mohamed provides a the hand amputation for theft law, and a message of forgiveness from Allah upon repentence.
After that, at verses 41-46, Mohamed attacks the apostacy of the Jews, accusing them of failing to follow the Torah, and especially of failing to follow Jesus.
Then, at verses 47-50, Mohamed attacks the alleged apostacy of the Christians, accusing them of failing to follow the Evangel - the gospels, and especially the law.
Then, at verses 51-63, Mohamed exhorts Muslims not to befriend Christians and Jews, and provides some rather stern condmenation of them.
Verses 64-65 address the Jews failure to follow the Torah.
Verses 66-68 address both the Jews and the Christians.
Verse 69 suggests that if the Jews, Christians and Sabellians "believe ... and [do] good-- they shall have no fear nor shall they grieve."
Verses 70-71 condemn the Jews for killing the prophets.
At verses 72-81, Mohamed launches a diatribe against Christians for believing in Christ's divinity and for accepting trinitarianism, which Mohamed (as will be seen later) mistakenly thinks is a trinity of the Allah, Jesus, and Mary.
"Certainly they disbelieve who say: Surely Allah, He is the Messiah, son of Marium; and the Messiah said: O Children of Israel! serve Allah, my Lord and your Lord. Surely whoever associates (others) with Allah, then Allah has forbidden to him the garden, and his abode is the fire; and there shall be no helpers for the unjust. Certainly they disbelieve who say: Surely Allah is the third (person) of the three; and there is no god but the one God, and if they desist not from what they say, a painful chastisement shall befall those among them who disbelieve. Will they not then turn to Allah and ask His forgiveness? And Allah is Forgiving, Merciful. The Messiah, son of Marium is but an apostle; apostles before him have indeed passed away; and his mother was a truthful woman; they both used to eat food. See how We make the communications clear to them, then behold, how they are turned away. Say: Do you serve besides Allah that which does not control for you any harm, or any profit? And Allah-- He is the Hearing, the Knowing. Say: O followers of the Book! be not unduly immoderate in your religion, and do not follow the low desires of people who went astray before and led many astray and went astray from the right path. Those who disbelieved from among the children of Israel were cursed by the tongue of Dawood and Isa, son of Marium; this was because they disobeyed and used to exceed the limit. They used not to forbid each other the hateful things (which) they did; certainly evil was that which they did. You will see many of them befriending those who disbelieve; certainly evil is that which their souls have sent before for them, that Allah became displeased with them and in chastisement shall they abide. And had they believed in Allah and the prophet and what was revealed to him, they would not have taken them for friends but! most of them are transgressors."
In verse 82, Mohamed even goes so far as to refer to Christians as polytheists: "Certainly you will find the most violent of people in enmity for those who believe (to be) the Jews and those who are polytheists, and you will certainly find the nearest in friendship to those who believe (to be) those who say: We are Christians; this is because there are priests and monks among them and because they do not behave proudly."
Next, there are a number of additional laws.
Then, from verses 109-115 there is a fictional dialog with Jesus, culminating in verse 116: "And when Allah will say: O Isa [Jesus] son of Marium [Mary]! did you say to men, Take me and my mother for two gods besides Allah he will say: Glory be to Thee, it did not befit me that I should say what I had no right to (say); if I had said it, Thou wouldst indeed have known it; Thou knowest what is in my mind, and I do not know what is in Thy mind, surely Thou art the great Knower of the unseen things."
This confirms the analysis above, that Mohamed was attempting (errantly) to attack the Christian doctrine of Trinitarianism, while mistakenly identifying Mary as the third person of the Trinitiy.
The remaining four verses of the Surah conclude the fictional dialog with Jesus, which emphasizes obedience to the law allegedly given by Allah.