There is a prominent Catholic apologist who asserts that Jesus was not referring to His life or death or act of salvation when He said, “It is finished”.
Here is an excerpt…
I was listening to my favorite pastor and preacher who also happened to be my Hebrew instructor and Old Testament professor. He was going through the Gospel of John, and he was focusing on Christ’s passion and death. Then he got to chapter 19, and then he came to those famous verses in John 19, beginning in verse 28, “After this, Jesus knowing that all was now finished said to fulfill the scripture, ‘I thirst.’ A bowl full of sour wine stood there, so they put a sponge full of sour wine on hyssop and held it to his mouth. When Jesus had received the sour wine, he said, ‘It is finished’ and he bowed his head and gave up his spirit.”
In the middle of his sermon, he was distracted by these words, “It is finished.” You could tell that he was taking in unplanned tangent. He said, “You know, well what exactly did Jesus mean when he said, 'It is finished?” Now this pastor always had a way of asking provocative questions, off the cuff, and then giving you brilliant answers off the cuff, that just opened up layer after layer of meaning for scripture. So I was sitting there waiting with bated breath for some mind-numbing insight, when all of a sudden he shocked me and said, “I’m not really sure what Jesus meant when he said, ‘It is finished.’ What is the ‘it’ that was finished?”
I’m sitting there thinking, “Come on, it’s Christ’s redemptive death.” And he said, “If you’re sitting there thinking it’s Christ’s redemption that is finished, you have to realize that the work of redemption was not completed with his death. As St. Paul says, ‘He was raised for our justification.’ So the resurrection is essential for our redemption every bit as much as the crucifixion. All right then, what did he mean when he said, ‘It is finished.’?” I just kept sitting there waiting until finally he said, “I’m not sure. Let’s just move on.”
He goes on to detail his study that resulted in his becoming Catholic.