No miracle can prove that immoral teaching is sacred. But it can prove that it is supernatural. And this is precisely what Scripture always proclaims. In the New Testament, we are bidden to take heed, because a day will come, when false prophets shall work great signs and wonders, to deceive, if possible, even the elect (Mark xiii. 22). In the Old Testament, a prophet may seduce the people to worship other gods, by giving them a sign or a wonder which shall come to pass, but they must surely stone him: they must believe that his sign is only a temptation; and above whatever power enabled him to work it, they must recognise Jehovah proving them, and know that the supernatural has come to them in judgment, not in revelation (Deut. xiii. 1-5).
Now, this is the true function of the miraculous. At the most, it cannot coerce the conscience, but only challenge it to consider and to judge.
A teacher of the purest morality may be only a human teacher still; nor is the Christian bound to follow into the desert every clamorous innovator, or to seek in the secret chamber every one who whispers a private doctrine to a few. We are entitled to expect that one who is commissioned directly from above will bear special credentials with him; but when these are exhibited, we must still judge whether the document they attest is forged. And this may explain to us why the magicians were allowed for awhile to perplex the judgment of Pharaoh whether by fraud, as we may well suppose, or by infernal help. It was enough that Moses should set his claims upon a level with those which Pharaoh reverenced: the king was then bound to weigh their relative merits in other and wholly different scales.