There is a quote popularly (and falsely) attributed to St. Augustine of Hippo which runs as follows:
“If you deny the trinity you lose your soul, if you try to explain the trinity you shall lose your mind,”
Of course Augustine never said this (in fact, the earliest reference I can find to anybody saying it is in the mid 20th century, and even this does not contain these words verbatim). Many well meaning Muslims, in engaging with Christians, have used this quotation (falsely attributing it to Augustine) when discussing the doctrine of the Trinity. Since I’m reviewing the doctrine for my upcoming exams, I thought I might take a moment to write a short post about why adopting this rhetoric is imprudent on the part of Muslims who wish to object responsibly to Christianity. Most Muslims disbelieve in the Trinity for two complementary reasons: (1) the doctrine is (allegedly) incompatible with monotheism (or, at least, those who believe it commit shirk), and (2) the Qur’an and sacred sources for Islamic doctrine (allegedly) explicitly reject, and/or are implicitly incompatible with, the doctrine of the Trinity. I do not think that either reason can stand up to serious scrutiny.
The first thing to note, in defense of the doctrine of the Trinity, is that it is not flatly contradictory. Christians are careful to distinguish the predicates ‘being’ (or ‘essence’ or ‘nature’) and ‘person,’ and consistently affirm that God is one being, while being three persons. This distinction alone is enough to guarantee the narrowly-logical possibility of the doctrine (as opposed to the broadly-logical or ‘metaphysical’ possibility). To put it somewhat formally, ∼(Bx⊃∼Tx) where B stands for ‘is one being’ and T stands for ‘is three persons.’ There is no strict contradiction in affirming that x is multiple beings and one person, or in affirming that x is one being and one person, or in affirming that x is one being and multiple persons. In order to argue that the latter claim is metaphysically impossible one must do a bit more work than merely expressing one’s incredulity; it will, in fact, have to be established that ‘if x is one being, then x is at most one person.’ It is, however, hard to see how to establish this, and it is not philosophically impressive to merely appeal to raw intuitions (that’s not to say it’s philosophically illegitimate, but just that it isn’t likely to persuade Christians).
Can the Christian do any better than appealing to her intuitions, or, perhaps, eschewing some of her intuitions in the interest of preserving her faith? I think so. In fact, in order to support the metaphysical possibility of the Trinity the Christian can appeal to its conceptual possibility. In other words, in order to provide respectable grounds for affirming the metaphysical possibility of the doctrine of the Trinity it plausibly suffices to demonstrate its conceptual possibility, which will, in turn, be satisfied just in case there is a logically possible world in which a single being is more than one person. This, however, is trivially easy; some social Trinitarians, like William Lane Craig, use the example of Cerberus (the three headed dog from Greek mythology standing guard at the gates of the underworld – or, for Harry Potter fans, Hagrid’s pet ‘Fluffy’ guarding the entrance to the world underneath Hogwarts). Clearly Cerberus exists in a logically possible world, and if his three centers of consciousness can (logically possibly) qualify as persons then Cerberus’ conceptual possibility is enough to justify inferring the metaphysical possibility of a single tri-personal being. I am not here endorsing Craig’s (or anyone’s) social trinitarianism, but borrowing his illustration to make a point about the conceptual possibility of the doctrine of the Trinity, which lends support to the belief that the doctrine is metaphysically possible.
Once it has been established that the Trinity does not strictly and logically entail tri-theism (or anything else logically incompatible with monotheism), Christians can justify their belief in the doctrine’s truth on a number of grounds. They can appeal to arguments for thinking that Christianity is true, or that the revelation in the Bible is trustworthy, and reasons like that. This is a tactic to which the Muslim should, I think, be sympathetic; after all, if the Qur’an taught the doctrine of the Trinity Muslims would (probably) faithfully believe it, precisely because they trust God, and they trust that the Qur’an is a reliable source of revelation. Thus, even if Christians are wrong about the Trinity, Muslims should be able to sympathize with their reasons for accepting that the Trinity is true.
Christians can go further than this, however, and follow the example of Tertullian in affirming that “prorsus credibile est, quia ineptum est” (roughly: “it is by all means to be believed, because it is absurd”). The idea here is that the doctrine of the Trinity is prima facie absurd, and it is absurd in precisely the way we would never expect of a man-made religious doctrine. Man-made religions are naturally made to be palatable to their original audiences (thus, to use Mormonism as an example, it is not very surprising that Mormonism democratizes divinity by distributing it among indefinitely many of us given that Mormonism came from a time and place where democracy was highly regarded). Christianity, coming from the bosom of first-century Judaism, and deeply acquainted with the world of Hellenistic philosophy since its nascency, had no impetus for manufacturing such a jarring doctrine, and plenty of impetus for rejecting it. There was, in fact, an incredible amount of pressure for the Church to reform her view, and many ways of explaining the Trinity have cropped up which looked philosophically consistent (eg. Sabellianism, Patripassianism, Adoptionism, etc.), but were nevertheless deemed heretical. Christianity’s insistence upon this counter-intuitive doctrine, especially while it regarded human reason as highly valuable, is extremely difficult to explain on the assumption that Christianity was (or became) a man-made credo. Moreover, Christians often argue that we should expect God’s nature, being so radically far removed from our own (or the nature of any created thing with which we are acquainted), to be hard or impossible for us to fully grasp (see Surah 23:92). We ought to expect God’s nature to be paradoxical, but not contradictory – and this balance is nowhere better achieved than in the Christian doctrine of the Trinity. The doctrine of the Trinity is not the kind of thing anyone would have made up, it is precisely the kind of thing man naturally wants to break into comprehensible bits – and this is precisely what should have happened on the assumption that Christianity has evolved without the oversight of the Holy Spirit. The Christian can find, in these considerations, a solid basis on which to sensibly say credo quia absurdum.
The Christian could add to these reasons by appealing to epistemological accounts of warrant which might justify belief in the Trinity on the basis of its being properly basic, or they might try to give some a priori or a posteriori arguments for the Trinity (such as I’ve done elsewhere). However, let’s not belabour this point – suffice it to say that Christians have some grounds for their belief in the Trinity to which Muslims ought to be sympathetic, regardless of whether they find those grounds entirely acceptable. Clearly belief in the Trinity is not incompatible with a commitment to monotheism; the Trinity does not entail the negation of monotheism, and belief in the Trinity does not psychologically commit one either to denying or failing to affirm monotheism.
Is it true that the Qur’an (or Hadith, or Sunnah) explicitly reject the Trinity? Not quite. Typically, the following passages are cited:
“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.” (Surah 5:73, trans. Yusufali)
“Allah has not taken any son, nor has there ever been with Him any deity. [If there had been], then each deity would have taken what it created, and some of them would have sought to overcome others. Exalted is Allah above what they describe [concerning Him].” (Surah 23:91)
“People of the Book, do not go to excess in your religion, and do not say anything about God except the truth: the Messiah, Jesus, son of Mary, was nothing more than a messenger of God, His word, directed to Mary, a spirit from Him. So believe in God and His messengers and do not speak of a ‘Trinity’—stop, that is better for you—God is only one God, He is far above having a son, everything in the heavens and earth belongs to Him and He is the best one to trust.” (Surah 4:171).
“Say: He is Allah, the One and Only; Allah, the Eternal, Absolute; He begetteth not, nor is He begotten; And there is none like unto Him.” (Surah 112:1-4).
It is imperative to keep in mind that while the doctrines of the Trinity and the Incarnation are joined at the hip in Christian theology, neither doctrine strictly logically implies the other. The Trinity could be true while the incarnation false, and vice-versa. Therefore, verses attacking the doctrine of the incarnation can be set to one side. Nevertheless, it still looks pretty damning for the doctrine of the Trinity, but here is a surprising reason to think it isn’t as bad as it appears; in Surah Al-Ma’idah we find the following passage:
“And behold! Allah will say: “O Jesus the son of Mary! Didst thou say unto men, worship me and my mother as gods in derogation of Allah’?”…” (Surah 5:116)
This verse puts all of the other verses into sharper historical and theological perspective. It is the only place in the Qur’an where the doctrine of the trinity being condemned is actually outlined. Oddly, this doctrine looks nothing like the Catholic/orthodox doctrine of the Trinity in which Christians believe. One hypothesis invoked to explain this oddity appeals to the heretical beliefs of the Collyridians. What we know about the Collyridians isn’t extensive, and comes exclusively from Epiphanius of Salamis. We do, however, know that they worshipped (rather than merely venerated) Mary, treating her as a goddess, and that they were chiefly located in the Arabian Peninsula. Although there is controversy about whether the group existed at the time of Muhammad (though we do know it existed in the 4th and 5th centuries), some have speculated that Muhammad became acquainted with their belief in the deity of Mary (either through immediate acquaintance, or through oral (or written) traditions he may have come in contact with). We do know that Muhammad had some familiarity with other gnostic groups and/or traditions (as evidenced, for instance, in the examples of the parallels between Surah 3:46-49, Surah 19:28-34, Surah 5:110, the Arabic ‘Gospel of the Infancy of Jesus Christ,’ and the ‘Gospel of Thomas the Israelite’), so it is entirely possible that Muhammad was exposed to the Collyridian doctrine and, under its influence, misunderstood the doctrine of the trinity.
If the Collyridian worship of Mary is what led to this confusion in the Qur’an (i.e., that the trinity consists of ‘God, Jesus and Mary’) then the statements in the Qur’an condemning the trinity it refers to come nowhere near offering a condemnation for the authentically Christian doctrine. In fact, whatever led to the confusion, the trinity being imagined in the pages of the Qur’an is obviously nothing like the Christian doctrine. First, God cannot be part of the Trinity, for the whole Trinity is God. Second Mary is nowhere ever considered part of the Trinity in Catholic/orthodox doctrine, nor has she ever been at any time. So even if the Collyridian heretics aren’t to blame, the Qur’an’s view of the trinity is not even in the same neighborhood as the Christian doctrine.
Although some sources have argued that something like the doctrine of the Trinity is implied in the Qur’an (e.g., because Allah constantly uses the first person plural attributing a plurality to himself, and for other reasons), I do not think that these arguments are very good. However, I do think that their very existence helps to reinforce the more general point that the Qur’an is not strictly opposed to the (Catholic) doctrine of the Trinity. Thus, even if the Qur’an doesn’t commit Muslims to anything like the Trinity, nothing about it is incompatible with the doctrine – indeed, strictly speaking, Islam and the (genuine) doctrine of the Trinity could both be true.
Finally, the Qur’an continually distinguishes Christians from polytheists, but this is difficult to make any sense of if the doctrine of the Trinity implies tri-theism. Christians are never even referred to as Mushrik or mušrikūn (مشركون)(those who commit the sin of Shirk) in the Qur’an. The Qur’an itself, therefore, assumes that Christians (people of the book, or ′Ahl al-Kitāb) are monotheists even though they believe in the Trinity, and thus that their form of Trinitarianism (even if wrong) is not, when properly understood, logically incompatible with monotheism.
Recall that Surah al-Ankabut says: “Do no argue with the people of the Book except in what is better… and say we believed in what was sent down to us and to you, and our God and your God is the same, is one” (Surah 29:46). Thus, the Christian doctrine of God (as trinity) is not so at odds with the teaching of Muhammad that he couldn’t declare the Christian God and the Muslim God to be the very same. Either the final words ‘is one’ mean ‘are both identical’ or it literally means that the Christian God ‘is [essentially] one,’ either of which vindicates (or excuses) the Trinitarian doctrine. Even on the assumption that the doctrine of the Trinity is wrong, if even the Qur’an excuses this belief, so too should Muslims.
In conclusion, taking all these points into consideration I think it is reasonable to maintain that Muslims ought to be more sympathetic towards the Christian belief in the Trinity than they typically are.
 Harold Lindsell and Charles J. Woodbridge, A Handbook of Christian Truth, (Westwood, NJ: F. H. Revell, 1953), pp. 51-52. I owe this reference to: http://scriptoriumdaily.com/who-said-the-trinity-try-to-understand-it-and-youll-lose-your-mind/
 William Lane Craig, “Toward a Tenable Social Trinitarianism,” Philosophical & Theological Essays on the Trinity edited by Thomas McCall and Michael C. Rea, (Oxford University Press, 2009): 98.