Certainly they are not the same doctrine. For one thing, Plotinus was, of course, a pagan rather than a Christian. (His student Porphyry was notoriously hostile to Christianity.) His position purports to be founded on purely philosophical arguments, whereas the Trinity is, according to Christian doctrine, something accessible only through divine revelation. Indeed, the Trinity is said to be a mystery – not in the sense that it is in itself in any way contrary to reason, but rather in the sense that the unaided human intellect is too limited either to arrive at it on its own or fully to understand it even once it has been divinely revealed. For another thing, many of the details of Plotinus’ conception of the three hypostases and their relations are simply incompatible with what the doctrine of the Trinity says about the three divine Persons. For example, the relationship between Plotinus’ hypostases is evidently subordinationist, whereas the Persons of the Trinity are equal in divinity.
All the same, mightn’t one argue that doctrines like that of Plotinus are at least pointers to the Trinity? Not that his views pointed historically to it – Trinitarianism was well established before Plotinus, even though the crucial formulation associated with the Council of Nicea would not be arrived until over half a century after his death. The suggestion on the table is rather that, considered as a piece of natural theology, Plotinus’ position points philosophically to something like the Trinity. The idea would be this: The human intellect cannot arrive at the doctrine of the Trinity per se, but it can discover grounds for suspecting that there must in some way be even more to the “personal” aspect of the divine nature than God’s having intellect and will – like the congenitally blind man who might suspect even apart from the testimony of the sighted that there is more to the physical world than what his four working senses tell him, even if he cannot in principle grasp what it is.
Nor need such a judgment be based merely on the fact that Plotinus posits exactly three divine hypostases. Consider Aquinas’s account of the Trinity in Summa Theologiae Part I, Questions 27-43. (Brian Davies provides a useful summary of Aquinas’s position in The Thought of Thomas Aquinas.) Aquinas compares the Father’s begetting of the Son to the intellect’s forming a concept of itself; and he compares the proceeding of the Holy Spirit to the willing or loving of this concept that the intellect knows. As we have seen, intellect and the directedness toward an object that love or desire manifest provided Plotinus with ways of characterizing the second and third of his hypostases.
What is missing from Plotinus’ account to prevent it from dovetailing more completely with the doctrine of the Trinity – or so this line of thought we are considering might continue, anyway – is Aquinas’s doctrine of analogy. It is because Plotinus is so emphatic about the absolute simplicity of the One that he must make of Intellect and Soul derivative hypostases rather than anything like distinct Persons in one divine substance. For he refuses to make any distinctions at all within the One itself. And yet Plotinus also affirms that the Forms are in the One, even if only “virtually”; and they must be there if the One is to explain the rest of the world, for a cause cannot give what it does not in some sense have to give. Hence Intellect and Soul are also there virtually. But if they are there even virtually, they are there. The doctrine of analogy, which informs Aquinas’s understanding of divine simplicity, can help us understand how this can be so. For Aquinas, there is in God something analogous to power in us and something analogous to knowledge in us; but where in us power and knowledge are distinct, in God they are the same thing, and it is precisely the fact that we are attributing power and knowledge to God in an analogous rather than a univocal sense that makes this intelligible. Similarly, perhaps we could say that what exist in a distinct way outside the One – the Forms, but also (more to the present point) Intellect and Soul – exist within the One as one, different only in our (analogous) descriptions of them, but not different in themselves. And in that way (or so it might seem) we can think of One, Intellect, and Soul as united in a way reminiscent of the relationship between Father, Son, and Holy Spirit as three Persons in one divine substance.
Does this proposed line of thought show, then, that Plotinus’ doctrine is a philosophical “pointer” to the doctrine of the Trinity? Not so fast. For one thing, any attempt to reconcile Plotinus with Aquinas via the doctrine of analogy can only be taken so far. It is true that the doctrine of analogy shows how we can predicate power, knowledge, goodness, etc. of God in a manner consistent with divine simplicity. But we cannot say that Father, Son, and Holy Spirit are identical in the way we can say that God’s power, knowledge, and goodness are identical, on pain of confounding the divine Persons and lapsing into heresy. Hence, whatever we might get by tucking up Intellect and Soul together with the One via the doctrine of analogy, it isn’t the Trinity.
For another thing, even if it is arguable that Plotinus’ doctrine nevertheless “points” to the Trinity in some vague way, this is something the Trinitarian himself would have to insist could not possibly be recognized except “after the fact.” That is to say, there can be no question of some philosopher who has no familiarity with Christian teaching on the Trinity independently discovering it, or even discovering something that “points” to it in anything but the vaguest way. For if that were possible, the Trinity wouldn’t be a mystery, wouldn’t be something we couldn’t possibly know of apart from divine revelation – it would instead be just one more piece of natural theology among others.
Skeptics might regard this emphasis on mystery as a shameless ploy to ward off potential criticism, but in fact it is, when properly understood, quite clearly the opposite of that. Trinitarian theologians insist that the doctrine is not contrary to reason – that it is perfectly coherent, and that every skeptical attempt to prove otherwise accordingly must be, and can be, answered. They do not claim that attempts to demonstrate the doctrine’s incoherence can legitimately be waved away by shouting “mystery!” Given this apologetic interest, it might seem they would welcome any purported purely philosophical demonstration of the Trinity that removes all mystery, as a means of shutting the skeptics up for good. But in fact they are hostile to such attempts, and Trinitarian orthodoxy requires them to be. The Trinitarian position is, as I have said, that the doctrine is in itself perfectly in harmony with reason but also that our intellects are too limited to grasp it. This entails both an insistence on defending the doctrine in a negative way, by rebutting attempted refutations, and an insistence that positive purely philosophical demonstrations of its truth are impossible.
Would-be scoffers should keep in mind that naturalists like Colin McGinn have taken a position similar to this as a way of staving off dualist objections, claiming that there is a purely naturalistic explanation of consciousness but that our minds are constitutionally incapable of understanding that explanation. They’ve even labeled this view “mysterianism.” Only someone who assumes that, if there really is a God, then the divine nature simply must be fully comprehensible to us, can object in principle to the parallel view defended by the Trinitarian. And why would anyone make such a foolish and ungrounded assumption?