One of themselves, even a prophet of their own, said, The Cretians are alway liars, evil beasts, slow bellies.
[One of themselves, even a prophet of their own] This was Epimenides, who was born at Gnossus in Crete, and was reckoned by many the seventh wise man of Greece, instead of Periander, to whom that honour was by them denied. Many fabulous things are related of this poet, which are not proper to be noticed here. He died about 538 years before the Christian era. When Paul calls him a prophet of their own, he only intimates that he was, by the Cretans, reputed a prophet. And, according to Plutarch (in Solone), the Cretans paid him divine honours after his death. Diogenes Laertius mentions some of his prophecies: beholding the fort of Munichia, which guarded the port of Athens, he cried out: "O ignorant men! if they but knew what slaughters this fort shall occasion, they would pull it down with their teeth!" This prophecy was fulfilled several years after, when the king, Antipater, put a garrison in this very fort, to keep the Athenians in subjection. See Diog. Laert., lib. 1 p. 73.
Plato, De Legibus, lib. 2, says that, on the Athenians expressing great fear of the Persians, Epimenides encouraged them by saying "that they should not come before ten years, and that they should return after having suffered great disasters." This prediction was supposed to have been fulfilled in the defeat of the Persians in the battles of Salamis and Marathon.
He predicted to the Lacedemonians and Cretans the captivity to which they should one day be reduced by the Arcadians. This took place under Euricrates, king of Crete, and Archidamus, king of Lacedemon; vide Diog. Laert., lib. 1 p. 74, edit. Meibom.
It was in consequence of these prophecies, whether true or false, that his countrymen esteemed him a prophet; that he was termed aneer (grk 435) theios (grk 2304), a divine man, by Plato; and that Cicero, De Divin., lib. 1, says he was futura praesciens, et vaticinans per furorem: "He knew future events, and prophesied under a divine influence." These things are sufficient to justify the epithet of prophet, given him here by Paul. It may also be remarked that vates and poeta, prophet and poet, were synonymous terms among the Romans.
[The Cretians are always liars] The words quoted here by the apostle are, according to Jerome, Socrates, Nicephorus, and others, taken from a work of Epimenides, now no longer extant, entitled Peri chreesmoon, Concerning Oracles. The words form a hexameter verse:
Kreetes aei pseustai, kaka theeria, gasteres argai.
The Cretans are always liars; destructive wild beasts; sluggish gluttons.
That the Cretans were reputed to be egregious liars, several of the ancients declare; insomuch that Kreetizein, to act like a Cretan, signifies to lie; and chreesthai Kreetismoo, to deceive. The other Greeks reputed them liars, because they said that among them was the sepulchre of Jupiter, who was the highest object of the Greek and Roman worship. By telling this truth, which all others would have to pass for a lie, the Cretans allowed that the object of their highest admiration was only a dead man.
[Evil beasts] Ferocious and destructive in their manners.
[Slow bellies.] Addicted to voluptuousness, idleness, and gluttony; sluggish or hoggish men.
This witness is true. Wherefore rebuke them sharply, that they may be sound in the faith;
[This witness is true.] What Epimenides said of them nearly 600 years before continued still to be true. Their original character had undergone no moral change.
[Rebuke them sharply] Apotomoos (grk 664). Cuttingly, severely; show no indulgence to persons guilty of such crimes.
[That they may be sound in the faith] That they may receive the incorrupt doctrine, and illustrate it by a holy and useful life.
(from Adam Clarke Commentary)