Obviously, this verse is against any mention of a "trinity" and divinity of Jesus. He was a blessed messenger, by GOD's will. At the same time, he worried about his own necks, because he was a human being, as all messengers are.
The Truth in the Koran
Defining the term
الحق = alhaqqi
alhaqqi definition:
The Truth; one of the names of allah ; due share; justice; right claim; what ought to be; duty; incumbent.
It comes from the root haqqa, which means it was, or became, suitable to the requirements of wisdom, justice, right or rightness, truth, or reality, or fact; or to the exigencies of the case; it was, or became, just, proper, right, correct, or true; authentic, genuine, sound, valid, substantial, or real; established, or confirmed as truth or fact; and necessitated, obligatory, incumbent or due; it was, or became, a manifest and an indubitable fact or event; it happened, betided, or befell, surely, without doubt or uncertainty. To be right, just or fitting, worthy of, justly due to, proper, genuine, real, a fact, true, necessitated, suitable, necessary, incumbent upon, suited to the requirement of justice, become certain, authentic, deserve.
References:
An Arabic-English Lexicon, E.W. Lane, volume two, pp. 605 610
The Dictionary of the Holy Quran, 1st edition, Abdul Mannan Omar p. 131
Per the classic definition, alhaqqi means 'The Truth', and is actually a name for allah.
Observe how alhaqqi is applied to Jesus Christ in the Koran
The Truth is only Jesus Christ (4.171)
The Truth is The Word (6.73, 4.171)
The Truth created the Universe (6.73, 6.101, 10.5, 14.19, 15.85, 16.3, 29.44, 39.5, 45.22, 46.3, 64.3)
The Truth paid the debt and is the sacrifice (48.28 )
The Truth forgives sins (47.2)
The Truth is Muhammad (47.2)
Applied to 4.171
يأهل الكتب لا تغلوا في دينكم ولا تقولوا على الله
إلا الحق إنما المسيح عيسى ابن مريم رسول الله
وكلمته ألقيها إلى مريم وروح منه فءامنوا بالله
ورسله ولا تقولوا ثلثة انتهوا خيرا لكم إنما الله
إله وحد سبحنه أن يكون له ولد له ما في
السموت وما في الأرض وكفى بالله وكيلا
Ya ahla alkitabi la taghloo fee deenikum wala taqooloo AAala Allahi illa alhaqqa innama almaseehu AAeesa ibnu maryama rasoolu Allahi wakalimatuhu alqaha ila maryama waroohun minhu faaminoo biAllahi warusulihi wala taqooloo thalathatun intahoo khayran lakum innama Allahu ilahun wahidun subhanahu an yakoona lahu waladun lahu ma fee alssamawati wama fee al-ardi wakafa biAllahi wakeelan
4.171 You The Book's family, do not go beyond the limits in your faith, and they do not say on allah except The Truth (is) only The Messiah Jesus, Mary's son, allahs messenger, and his Word, cast forth to her, Mary, and Spirit from him; so believe by allah, and his messengers, and they do not say "Three." Refrain (it is) certainly agreeable to you, only allah one god glory be to him, that He has certainly been his Son, truly His what is in the heavens and in the earth and He sufficed by allah, a witness.
Here is the classic definition for innama
إنما = innama
innama definition:
According to the grammarians, it is a compound of inna and ma, which latter prevents the formers having any government: it imports restriction: it imports the restriction of that which it precedes to that which follows it. In other words, it is used to particularize, or specify, or distinguish a thing from other things: it affirms a thing in relation to that which is mentioned after it, and denies it in relation to other things. Some say that it does not import restriction, but only corroboration of an affirmation, because it is a compound of the corroborative inna and the redundant ma which restrains the former from exercising government, and that it has no application to denote negation implied in restriction. It therefore seems that it is susceptible of both these meanings, bearing one or the other according as this or that suits the place. Rendered as only, verily.
References:
An Arabic-English Lexicon, E.W. Lane, volume one, pp. 103 111; 118
A Grammar of the Arabic Language, W. Wright, Third edition, volume 1, p. 285
The Dictionary of the Holy Quran, 1st edition, Abdul Mannan Omar, p. 35
A Dictionary and Glossary of the Koran, John Penrice, p. 11
The classic definition quite clearly states that innama affirms a thing in relation to that which is mentioned after it.
Thus, alhaqqi (i.e. The Truth), is the thing that is being related to Jesus which is mentioned after it.
Hence, The Truth is only Jesus Christ.