(I) Non-Christians cannot understand the Trinity.
(II) Christians always understand the Trinity.
(III) Accepting the Trinity is rational.
The purpose of my defense is to promote discussion about the Trinity doctrine's meaningful content, or lack thereof, by resisting objections to the effect that non-Christians cannot understand it.
My defense turns on the important status given to the Trinity doctrine by orthodox Christians. Indeed, with its long history of difficult formulation, the Trinity stands today as a particularly controversial doctrine in Christian theology: On one hand, many Christians consider it to be an essential tenet of faith, such that anyone who denies the Trinity is not, by their reckoning, a true Christian. Yet while educated Christians are often able to articulate the doctrine precisely and with basic agreement, many other Christians have considerable trouble expressing it, and find serious disagreement in its various aspects. It strikes me as strange, however, that an essential doctrine should appear so widely misunderstood within the Christian community. Having observed this, we might ask, if the Trinity is to be taken as an essential doctrine, then in what sense is it really essential, given that so many self-professed Trinitarian Christians cannot communicate it? Perhaps the key is not in acceptance, but rather denial: So, one may, for whatever reason, fail to accept the Trinity, and still be Christian; however, if one denies the Trinity, then he ought not be called Christian.
Actually accepting the doctrine of the Trinity, then, seems both unnecessary and insufficient for being a Christian. To illustrate this intuitively, we can consider four distinct types of people, by no means an exhaustive list, who do not accept the Trinity, but who neither reject it: Firstly, there are agnostics who claim to understand the Trinity, but who have declined to believe it for some other reason, for example the perceived lack of evidence for the Trinitarian God's existence. This shows rather unequivocally, and believers should for the most part be happy to agree, that merely declining to deny the existence of the Trinitarian God is not sufficient to define a Christian. Second, we have self-described Christians who claim to understand the Trinity, but who profess agnosticism as to whether or not it accurately represents the relationship between Jesus, the Father and the Holy Spirit. These Christians might, for instance, accept the divinity of Jesus without being sure how to properly reconcile that doctrine with monotheism. Thirdly, we may encounter Christians who freely admit that they do not understand the Trinity. Even if such Christians claim to be Trinitarian, they cannot possibly be so if their profession of ignorance is correct; for one cannot accept a doctrine, except by proxy, unless one understands it. Fourth, and finally for our purposes, many Christians who describe themselves as Trinitarian, and who believe they deeply appreciate the doctrine, in actuality have little or no understanding of it. We should not expect such Christians to be few in number, and indeed I suspect they are the most common of all four groups we have discussed thus far. As with those Christians who openly acknowledge their ignorance, these Christians whose non-understanding remains hidden, at least to themselves, cannot really accept the Trinity if they do not know what it entails. Certainly more deserves to be said regarding these types, especially for those Christians who fail to understand the Trinity, whether knowingly or not. For now, however, it suffices to have briefly surveyed various groups of people who do neither accept nor reject the Trinity, in order to show that they are not all obviously Christian or non-Christian.
If the Trinitarian acknowledges that not all Christians understand the Trinity, then hopefully this will enable him to consider the possibility that he himself hasn't properly understood it, either. In that case, we might seek discussion along those lines. Nevertheless, some Christians may insist that even if a person fails to deny the Trinity, if he does not accept it he cannot be a Christian. However, this view, when taken with certain uncontroversial assumptions, has serious consequences, in particular, that either non-Christians are able to understand the Trinity doctrine or else all Christians make an irrational decision when they accept the Trinity. For, if non-Christians are unable to understand the Trinity on one hand, and on the other hand Christians always accept (and therefore understand) the Trinity, then this means that a person only begins to understand the Trinity at the very instant of becoming Christian. So, there can be no time for rational deliberation, which of course means that any such decision to accept the Trinity must be irrational. Presumably the Christian will wish to preserve his sense of rationality, and so when confronted with this choice, should he recognize its necessity, he is likely to acknowledge that non-Christians are capable of understanding the Trinity. The conversation can thereby move forward to exploring that doctrine.
In case the Christian insists that non-Christians can never understand the Trinity, while simultaneously holding that anyone who does not accept the Trinity is not a true Christian, then we can drive home the point with a formalization of the afore-mentioned argument:
(1) A person x is a Christian at time t only if x accepts doctrine Y at t.
(2) Person x understands doctrine Y at time t only if x is a Christian at t.
(3) Person x makes a rational decision to accept doctrine Y only if x deliberates on doctrine Y.
(4) If person x deliberates on doctrine Y, then there is a time t at which x understands but does not accept doctrine Y.
(5) If person x accepts doctrine Y at time t, then either x makes a rational decision to accept doctrine Y, or else x makes an irrational decision to accept doctrine Y.
(6) Therefore, if person x is a Christian at time t, then x makes an irrational decision to accept doctrine Y.
The deductive validity of this argument may be verified using the following symbolic representation:
(1) (t)(x)(Cxt → Axt);
(2) (t)(x)(Uxt → Cxt);
(3) (x)(Rx → Dx);
(4) (x)(Dx → (∃t)(Uxt ∧ ¬Axt));
(5) (t)(x)(Axt → (Rx ∨ Ix));
(6) therefore (t)(x)(Cxt → Ix);
where Uxt symbolizes the statement "Person x understands doctrine Y at time t," Axt symbolizes "Person x accepts doctrine Y at time t," Cxt symbolizes "Person x is a Christian at time t," Rx symbolizes "Person x makes a rational decision to accept doctrine Y," Ix symbolizes "Person x makes an irrational decision to accept doctrine Y," and Dx symbolizes "Person x deliberates on doctrine Y." Clearly, we take "doctrine Y" in all cases to denote the Trinity, but the generalization allows us to apply the argument to any doctrine whose acceptance is perceived as essential to salvation.
Here we wish to show the consequences holding to both of the first two premisses. In contrast, premisses (3)-(5) seem fairly straightforward, though objections may be thrown up against these, as well. For example, a Christian might wish to quibble over premiss (5), claiming that there is a difference between irrational and non-rational decisions. Yet even if we allow such accommodations, we should still feel the force of the argument.
Recall again statements (I)-(III) from earlier: If a Trinitarian is willing to deny (III), then while it may end any discussion of the meaningfulness of the Trinity doctrine, it presents an opportunity for exploring the rationality or irrationality of Christianity, a worthy subject on its own. Otherwise the Christian can deny (I) or (II); if (I), then clearly the conversation can continue unabated; if (II), then, as mentioned previously, this motivates the Christian to reflect upon his own meaning, and we can foster care and attention in that task.