Deuteronomy 13:1

If....a prophet, or even an angel from heaven, as St. Paul (Galatians i. 8,) says on a similar occasion, (Calmet) should work a miracle, and afterwards adduce it in proof of a false religion, believe him not. The Jews and Christians had already received such convincing proofs from God, of the truth of what they had been taught, that they had reason to conclude either that the miracle was false, or that the person who would persuade them to embrace a different religion had fallen, after God had honoured him with miraculous powers: or, in fin, that if he were an impostor at the time when he exercised that power, like the magicians of Egypt, or Balaam, the miracle was either not wrought in confirmation of what he preached, or at least it was eclipsed by some greater miracle in favour of the truth. Whether God will ever suffer a real miracle which may seem to countenance error, or not, this appears to be unquestionable, that he will never deny himself, or, in a contest of miracles, permit falsehood to gain the victory. If the magicians performed wonderful works, they were forced at last to confess their inferiority, and yield to Moses, Exodus viii. 18, 19. Miracles are generally a proof of any doctrine; but when the doctrine is already established, as in this case of the unity of God, (ver. 2,) it may be adduced with propriety as a criterion of miracles. Truth can never be in contradiction to truth. The light of reason suffices to evince that there is but one God. The same truth had been repeatedly confirmed by miracles, particularly during the last forty years, during which God had manifested his power over all nature, in the sight of all the Hebrews, and had trampled on the idols of the Gentiles. If therefore any person should attempt, by his dreams or predictions, in invalidate this most fundamental and undeniable article, his testimony could not be received. (Haydock) --- The Jews, in vain, allege this passage against the religion of Jesus Christ. He did not subvert, but fulfilled the law; so far was he from endeavouring to persuade them to abandon the true God. (Calmet) --- If he had not come to act in this manner, the law would have contained in itself the seeds of dissolution, by falsely holding forth the expectation of a future Messias, who would bring all things to perfection, chap. xviii. 15., Genesis iii. 15., and xlix. 10, &c. Hence when he really appeared, the Jews desired him to prove his mission by a miracle, as he did repeatedly, Matthew xii. 38., John viii. 40., and x. 25. --- A dream, of a mysterious kind, like those of Joseph and of the prophets. (Haydock)