Salvation by Grace through Faith by Arlen L Chitwood and published by Classics.
When Christ referred to His finished work immediately before His death on the Cross, He cried out one word in "a loud voice" -Tetelestai - which has been translated in the English text, “It is finished” (John 19:30; cf. Matt. 27:50; Mark 15:37; Luke 23:46). Tetelestai is the perfect tense form of the Greek verb, teleo, which means “to bring something to an end or completion.” This word in the perfect tense could be more accurately expressed and translated, “It has been finished,” or “It has been completed.”
That to which Christ referred in John 19:30 was His work of redemption. The perfect tense which He used refers to a work completed in past time with the results of that work extending into and existing during present time in a finished state. At the moment Christ cried out, announcing that His work had been completed, there was then no reason for His death to be prolonged. The blood of the Passover Lamb had been shed, and God had "laid on him the iniquity of us all" (Isa. 53:5, 6, 12; cf. II Cor. 5:21). Thus, at this point, “he bowed his head, and gave up the spirit lit., comparing the other gospel accounts, ‘He breathed out,’ i.e., He expired, willingly giving up His life.” (John 10:17, 18; 19:30).
