Because Baha'u'llah said so:

Let us first distinguish two senses of saying that x is wrong because Bahá'u'lláh says it is wrong (where x is some activity, action, state of affairs, etc). First, one might simply be asserting that Bahá'u'lláh's saying that x is wrong gives us a (peremptory) reason for believing that it is so. Much of the rhetorical power of the because-Bahá'u'lláh-said-so school rests on this intuition. But this is besides the point when one is engaged in moral justification. When we demand the basis of the Bahá'í normative position on x, we are asking for (a) the philosophical justification of the position that x is wrong, and not for (b) the reason for the belief that it is so, or for the moral agent's reason for action according to that position.⁽²⁾

One may, for example, have very good reasons for believing that something is true, but without knowing the reasons why it is that it is true.

Second, one might be asserting that Bahá'u'lláh (or God) saying that morality is such and such makes it so. That is, one might adopt the position of divine voluntarism, which holds that the moral good is the moral good simply because it is willed as the moral good by the divine Will. Here then, one could reject the because-Bahá'u'lláh-said-so school by rejecting voluntarism. One would accuse the school of getting it backwards: x is not wrong because Bahá'u'lláh said it is wrong, rather, Bahá'u'lláh said it is wrong because it is wrong.⁽³⁾ But I wish to remain agnostic on this issue, and leave open the possibility, without committing myself to it, that Bahá'í ethics presupposes divine voluntarism. For example, one might cite, in favour of a voluntarist position, the passage in The Kitáb-i-Aqdas in which Bahá'u'lláh urges religious leaders not to judge the word of God by their own prevalent standards since God's word itself is the standard of right and wrong, truth and error: "the Book itself is the unerring Balance established amongst men" (¶99). This passage need not at all be taken to be one in support of the voluntarist position, but my argument here does not depend on settling that issue one way or the other.

If this passage is interpreted as supporting voluntarism, it could also be cited in favour of the because-Bahá'u'lláh-said-so school, but only if one interprets its application in the first of two ways. The first way is the doctrine of immediate application, the second a doctrine of mediate application. On the first doctrine, one would apply voluntarist justification immediately at the level of each and every particular injunction, ordinance, exhortation, law, and claim of Bahá'í ethics. So the moral reason that, say, taking mind-altering substances is wrong, would be that Bahá'u'lláh has forbidden it, period. We would need no further reflection on the matter, and the reasons given in the Bahá'í writings for the law, such as its effect on the soul and human moral agency, would be superfluous. Much of the rhetorical force of the doctrine of immediate application rests on the ambiguity, identified earlier, between two very different propositions. One was (a) the proposition that the divine say-so provides the justificatory basis for a given moral claim or principle; another was (b) the proposition that the divine say-so provides a (peremptory) reason for belief in, or a (peremptory) reason for action according to, that principle. The because-Bahá'u'lláh-said-so school, and its doctrine of immediate application, would have us believe that the second proposition (b) justifies the first (a), but the move from (b) to (a) is clearly logically fallacious.

In the case of the doctrine of mediate application, the (fiat of the) divine Will serves as a voluntarist justification for the totality of the ethical theory as a whole, but not as a justification for a particular moral or ethical claim. Instead, the justification for a particular moral claim is made in terms of the other moral principles and concepts which form a part of the total theoretical framework. On this doctrine, for example, Bahá'í ethical theory's justification for the particular moral claim that some action (e.g., taking mind-altering substances) is wrong, must be given in terms of the other moral principles and concepts which form the totality of Bahá'í ethics (e.g., it retards the progress of the soul, or it forfeits "moral responsibility"), even if the ultimate basis for that totality be the divine say-so. In other words, the divine say-so plays a role in meta-ethical, but not ethical, justification. The mediate doctrine presupposes, of course, that Bahá'í ethics forms a rationally coherent theoretical framework.

The point here is not that divine voluntarism is correct; rather that even if we were to concede it, we would still need to reject the immediate application doctrine, and with it the because-Bahá'u'lláh-said-so school, because a Bahá'í voluntarist account must adopt a mediate application doctrine. That this is so can be readily seen by the fact that both Bahá'u'lláh and 'Abdu'l-Bahá write as if their teachings form part of a coherent whole, and hence both freely justify a given principle (e.g. the forbidding of alcohol) in terms of other Bahá'í ethical principles and concepts (e.g. moral responsibility, or the progress of the soul). This also appears to be part of the reason why so many Bahá'ís have difficulty coming to terms with the fact of male-exclusivity in the membership of the Universal House of Justice: because it appears not to be justifiable in terms of, indeed it appears to contradict, other moral principles which form a part of the totality of Bahá'í moral and political philosophy. Notice again that citing Bahá'u'lláh's say-so regarding male-exclusivity only provides Bahá'ís with (b) a (peremptory) reason for believing that it is a proper moral principle⁽⁴⁾ (presumably fitting coherently into the whole theoretical framework), and a (peremptory) reason for action according to that principle, without illuminating (a) the justificatory basis for that principle--after all, 'Abdu'l-Bahá had said that that justificatory reason would become apparent in the future, even though Bahá'ís already had Bahá'u'lláh's say-so when 'Abdu'l-Bahá was writing. The latter does not provide the former.

So to determine the basis for the Bahá'í position on some question in ethics, one must consider Bahá'í ethical theory as a whole, and justify the position in those terms, and not in terms of the divine say-so. What is more, given the Bahá'í principle of the harmony of science and religion, and that religion must be scientific in its method, the Bahá'í position must be interpreted in light of some background knowledge gleaned from the natural and social sciences. If an interpretation of the Bahá'í position on racism were advanced which entailed or presupposed that the earth was flat, then that would count against the interpretation in question--insofar as the scientific evidence suggests that the earth is not flat.⁽⁵⁾

End Notes

1. This is a favourite rhetorical tool used when one is too fearful to hold one's own beliefs up to the light of consultative scrutiny, and wishes instead to silence others into submission.
2. For the reader not familiar with the terminology, I offer the following explication of a "reason for belief", a "reason for action", and a "peremptory" or "preemptive" reason, as these concepts are used in moral and political philosophy. For example, consider a possible claim in political philosophy that (1) a legitimate authority's commands give you a "peremptory reason for action", but (2) not a "peremptory reason for belief". This dual claim means the following. Let us say that there are a set of (say, moral) reasons why you ought to do x, and a set of other (moral) reasons why you ought not to do x, but do y instead (where x and y are distinct actions). And let us say that after careful reflection, you think the balance of moral reasons tells you that you ought to do x, rather than y. But then the Local Spiritual Assembly (LSA), after independent consideration, tells you that the balance of moral reasons means that you ought to do y, and commands you to do this. Now, on the account of the legitimacy involved in the dual claim we are examining, the LSA, as a legitimate authority, does not simply add one more reason to the balance of reasons in favour of doing y. Rather, (1) its authoritative command provides a reason for action that preempts - replaces and overrides - all other reasons in the previous balance of reasons that you were considering; i.e., you ought to do y. The LSA telling you that the balance of moral reasons requires y might also provide you with a reason for believing that the original balance of moral reasons was in favour of y after all. But the second part of the dual claim above says that (2) the LSA does not provide you with a peremptory reason for belief - in other words, you might still believe that the LSA's evaluation of the original balance of reasons was mistaken, but (1) your reason for action (in accordance with the command of the LSA) is independent of that belief, because the LSA's authority is legitimate.
         What is important about the above is not that one accepts the philosophical claim about the nature of (an LSA's) legitimacy; rather, what is important, for the purposes of this essay, is that one understands the difference between peremptory and non-peremptory reasons, and between reasons for action and reasons for belief. These distinctions are discussed more fully by Joseph Raz, The Morality of Freedom, Oxford: Clarendon Press, 1986.
3. Of course, this is an ancient debate, going at least as far back as Plato (cf. Euthyphro 10a).
4. A fallibilistic qualification is due even here. Because our understandings of the Writings are always subject to the fallible limitations of our own interpretations, this means that even those interpretations, which give rise to reasons for belief, must always be held to be subject to revision.
5. I should also like to make brief reference to appeals to "faith" in moral argument. Appeals to faith to settle moral questions have almost exactly the same structure as appeals to the divine say-so; indeed, this essay could have been written, in much the same form, about appeals to faith. Just as the faulty appeal to the divine say-so that I have tried to identify here often serves as a way of ending further moral reflection and consultation, so too does a faulty appeal to faith as the justification for some ethical claim. It is a fundamental Bahá'í belief, as I understand it, that faith has an important role in moral life. For example, just as appeals to the divine say-so may partially explain our reason for belief, an appeal to faith may also partially explain that belief, or even better, help explain our reason for action. But, as this essay has attempted to argue, neither an appeal to the divine say-so, nor an appeal to faith, plays a role in the philosophical justification of particular moral principles (as the doctrine of immediate application would have us believe). Such appeals might, on the other hand, partially account for one's commitment to an ethical theory as a whole, though this essay has failed to argue for a position on this question one way or the other.