"It Is Finished"
Messiah came to do the Father's will, which was all spelled out in the Torah and the Prophets.
Hebrews 10:7 Then Jesus said, "Behold, I have come—In the volume of the book it is written of Me—To do Your will, O God.”
Luke 24:44 Then Jesus said to them, “These are my words that I spoke to you while I was still with you, that everything written about me in the Law of Moses and the Prophets and the Psalms must be fulfilled.”
Messiah had to fulfill everything that was prophesied of Him all throughout the Scriptures. This is why He said this.
Matthew 5:17 Do not think that I came to destroy the Torah or the Prophets. I did not come to destroy but to fulfill them.
The final act of Messiah, in fulfilling the first part of His mission, was to be numbered with the transgressors.
Isaiah 53:12 Therefore, I will give him a portion among the great, and he will divide the spoils with the strong, because he poured out his life unto death, and was numbered with the transgressors. For he bore the sin of many, and made intercession for the transgressors.
Being number with the transgressors was Him being crucified between two thieves. His final act of dealing with the sins that were committed under the first covenant, was His death on the cross.
Hebrews 9:15 That is why he is the one who mediates a new covenant between God and people, so that all who are called can receive the eternal inheritance God has promised them. For Christ died to set them free from the penalty of the sins they had committed under that first covenant.
This is why He needed to be number with the transgressors.
Luke 22:37 It is written: ‘And he was numbered with the transgressors’; and I tell you that this must be fulfilled in me. Yes, what is written about me is reaching its fulfillment.”
Messiah saying, “It is finished” has nothing to do with Him obeying the law, so that we don’t have to.