A lying tongue is but for a moment - Truth stands for ever; because its foundation is indestructible: but falsehood may soon be detected; and, though it gain credit for a while, it had that credit because it was supposed to be truth.

The lip of truth shall be established for ever,.... The man that speaks truth is and will be established in his credit and reputation among men; he is uniform and all of a piece, and what he says is believed; truth, though it may be opposed, will prevail against lies and falsehood; the word of truth, the Gospel of Christ, will stand for ever; the ministers of truth and righteousness will be continued to the end of the world; Christ, who is truth itself, abides the same to day, yesterday, and for ever;
but a lying tongue is but for a moment; if a liar speaks truth for once, he does not continue in it long, but quickly returns to his former course; or rather the lie he tells is very short lived, it is soon discovered, and he comes into contempt and disgrace, and loses all his credit and reputation among men of honour and honesty, and is sometimes suddenly snatched away by death, as Ananias and Sapphira; all error and heresy in a short time will cease and be no more; and antichrist, whose coming is with lying wonders, the direct opposite of the lip of truth, will be brought to ruin in a moment. Aben Ezra, Kimchi, and Ben Melech, who are followed by some Christian interpreters, as Montanus, and some in Vatablus, render it, "but", or "for ever, I will cause the lying tongue to cease", or "be at rest" from speaking; as if they were the words of God, threatening to cut off the lying tongue; but Jarchi and Gersom render it "for a moment", or a very short time, as we and others do; or, "whilst one winks" (f), in the twinkling of an eye; so soon is such a person removed.
(f) "at dum nictem", Schultens.

If truth be spoken, it will hold good; whoever may be disobliged, still it will keep its ground.

Words of truth are consistent, and stand all tests, while lies are soon discovered and exposed.

19 The lip of truth endures for ever,
But the lying tongue only while I wink with the eye.
None of the old translators understood the phrase ועד־ארגּיעה; the Venet. also, which follows Kimchi's first explanation, is incorrect: ἕως ῥήξεως, till I split (shatter) it (the tongue). Abulwald is nearer the correct rendering when he takes ארגיעה as a noun = רגע with He parag. Ahron b. Joseph is better in rendering the phrase by: until I make a רגע, and quite correct if רגע (from רגע = Arab. raj', which is used of the swinging of the balance) is taken in the sense of a twinkling of the eye (Schultens: vibramen); cf. Orelli's Die hebr. Synonyme der Zeit und Ewigkeit, p. 27f., where the synonyms for a twinkling of the eye, a moment, are placed together. עד (properly progress) has in this phrase the meaning, while, so long as, and the cohortative signifies, in contradistinction to ארגיע, which may also denote an unwilling movement of the eyelids, a movement proceeding from a free determination, serving for the measurement of a short space of time, Ewald, 228a. ארגיעה, Jeremiah 49:19; Jeremiah 50:44, where Ewald takes כי ארגיעה (when I...) in the same sense as אד־ארגיעה here, which is more appropriate than the explanation of Hitzig, who regards כי as opening the principal clause, and attaches to הרגיע the quite too pregnant signification "to need (for an action) only a moment." The lip of truth, i.e., the lip which speaketh truth, endures for ever (for truth, אמת = אמנתּ, is just the enduring); but the tongue of falsehood is only for a moment, or a wink of the eye, for it is soon convicted, and with disgrace brings to silence; for a post-bibl. Aram. proverb says: קוּשׁטא קאי שׁקרא לא קאי, the truth endures, the lie endures not (Schabbath 104a), and a Hebrew proverb: השּׁקר אין לו רגלים, the lie has no feet (on which it can stand).
(Note: Vid., Duke's Rabbin. Blumenlese (1844), p. 231.)

A moment - Liars, though they may make a fair shew for a season, yet are quickly convicted.