It is done. It is finished. It is consummated.
You say that the doctrine of Purgatory implies a denial of what Jesus said on the cross: It is finished.
If what you say is true, then: I was baptized in 1953. But Jesus said “It is finished” about 28 AD. Therefore, by having me baptized, my parents were asserting that “it” was NOT finished.
To put it another way:
If the notion of the change or growth of people in Purgatory is precluded by Jesus’ words “It is finished,” then the notion of ANY form of change or spiritual growth in people is precluded by Jesus’ words, “It is finished.”
Every time a sinner turns away from sin, he is contradicting Jesus, because turning away from sin implies that the work of salvation is not finished.
Therefore, by having me baptized, my parents were asserting that “it” was NOT finished.
Hardly, they recognized that they are sinners, that you were conceived in iniquity and needed a Savior. Great parents. Sin wasn't finished at the cross because people still live on earth. But the price of sin has been paid. As another hymnist said 'death may no longer appall you.' Christians need not fear death, God has forgiven their sin for the sake of His Son Jesus and made them heirs with Him in His Kingdom.
because turning away from sin implies that the work of salvation is not finished.
No, it recognizes that sanctification isn't finished and that won't happen until heaven. When robed with Christ's righteousness before the throne of God, we stand free and guiltless. Not though our merit, our time in 'purgatory' but because of the work and righteousness of Christ. Humans will sin till death some more than others but all will sin. But that price has been paid, that is what Christ's cry of Tetelestai! means for us.
Just a play on words...The sacrifice is finished...The atonement is completed...The price is paid, in full...The penalty for sin is complete...