John Chrysostom (A.D. 349-407) wrote the following on the meaning of τετελεσται “it is finished” in John 19:30:
They parted the garments, by which such great miracles were done. But they wrought none now, Christ restraining His unspeakable power. And this was no small addition of insult. For as to one base and abject, as I said, and the vilest of all men; so do they dare to do all things. To the thieves at any rate they did nothing of the kind, but to Christ they dare it all. And they crucified Him in the midst of them, that He might share in their reputation.
And they gave Him gall to drink, and this to insult Him, but He would not. But another saith, that having tasted it, He said, "It is finished." And what meaneth, "It is finished?" The prophecy was fulfilled concerning Him. "For they gave me," it is said, "gall for my meat, and for my thirst they gave me vinegar to drink." But neither doth that evangelist indicate that He drank, for merely to taste differs not from not drinking, but hath one and the same signification.
But nevertheless not even here doth their contumely stop, but after having stripped and crucified Him, and offered Him vinegar, they proceeded still further, and beholding Him impaled upon the cross, they revile Him, both they themselves and the passers by; and this was more grievous than all, that on the charge of being an impostor and deceiver He suffered these things, and as a boaster, and vainly pretending what He said. Therefore they both crucified Him publicly, that they might make a show of it in the sight of all; and therefore also they did it by the hands of the soldiers, that these things being perpetrated even by a public tribunal, the insult might be the greater. (Homilies of John Chrysostom: Matthew XXVII.27-29, Homily LXXXVII, 1)
Hardly consistent with forensic atonement/penal substitution that many Protestants, especially Calvinists, desperately try to read into the text! To see a fuller discussion, see, for instance, Why Latter-day Saints cannot believe Evangelical Protestantism is True: A Response to Dave Bartosiewicz under the section entitled, "John 19:30."
The following video by a Roman Catholic apologist discusses John 19:30, showing, in part, the deceptive use of sources by Matt Slick and other Reformed apologists (e.g., their repeated but unsubstantiated claim about how τετελεσται's use in texts contemporary with the New Testament):