The Finished Work starts before the beginning and never ends because Jesus had no beginning and no end.
"I am Alpha and Omega, the beginning and the end, the first and the last." Revelation 22:13 KJV
"John to the seven churches which are in Asia: Grace be unto you, and peace, from him which is, and which was, and which is to come; and from the seven Spirits which are before his throne" Revelation 1:4 KJV
He was a lamb before the world began and He remains a wounded bloodless lamb far into the future.
"And I beheld, and, lo, in the midst of the throne and of the four beasts, and in the midst of the elders, stood a Lamb as it had been slain, having seven horns and seven eyes, which are the seven Spirits of God sent forth into all the earth... Saying with a loud voice, Worthy is the Lamb that was slain to receive power, and riches, and wisdom, and strength, and honour, and glory, and blessing." Revelation 5:6, 12 KJV
What He did on the cross is marvelous. It revealed who God was and is and is to come. Jesus is complete and His work would be complete.
"Men of Israel, hear these words: Jesus of Nazareth, a man attested to you by God with mighty works and wonders and signs that God did through him in your midst, as you yourselves know - this Jesus, delivered up according to the definite plan and foreknowledge of God, you crucified and killed by the hands of lawless men. God raised him up, loosing the pangs of death, because it was not possible for him to be held by it." Acts 2:22-24 ESV
God knew. God had a predetermined plan to reveal Himself in a certain way.
"For God so loved the world, that he gave his only Son, that whoever believes in him should not perish but have eternal life. For God did not send his Son into the world to condemn the world, but in order that the world might be saved through him. Whoever believes in him is not condemned, but whoever does not believe is condemned already, because he has not believed in the name of the only Son of God." John 3:16-18 ESV
Jesus was 'foreordained before the foundation of the world, but was manifest in these last times for you' (see 1 Peter 1:20). God revealed His grace by and through His Son. God was always gracious and will always be gracious. God used time as a canvas and left none of it blank. All of time is painted with His love.
Jesus is the lamb and in the Lamb's book names are written. Those wounds are fresh and lasting so are those names written forever in that book. The Book of life is another book. It is like a ledger. Names are written in it and taken out again. People live and then they die. The lamb's book is forever and the names there are never erased. One book chronicles human history and the other chronicles Jesus' history.
The Lamb died and received power, riches, wisdom, strength, honor, glory, and blessing. Those He won are trophies of His grace. Believers are the spoils of war. His victory is a complete victory.
The scope of the work spans all of time. The cross and consequential resurrection are points in time where that plan is revealed.
"But we impart a secret and hidden wisdom of God, which God decreed before the ages for our glory. None of the rulers of this age understood this, for if they had, they would not have crucified the Lord of glory." 1 Corinthians 2:7-8 ESV
The cross was a surprise to the onlooker. It would reveal what could only be revealed from the depth of a forever love. It revealed a love with no beginning and no end that covered everything a man could ever do. It was a complete covering. It is a finished work.