Hi
wondering,
Well right, of course. That's what is happening here. The OP is asking 'what' Jesus was referring to as the subject for 'it' in his call to his Father that 'It' is finished.
Now, I have read the Scriptures through a number of times and what seems obviously plain to me is that when God called Abram of Ur, through the promises and covenants that He made with Abram, God was setting into a motion a great plan that was going to be worked out upon the earth, first among His people, Israel, and then released as salvation unto the uttermost parts of the earth. It was a plan!
So Abram (Abraham) did his part and God did bless him and make his generations as uncountable as the sands of the seashore. But throughout all of those hundreds of generations of God's blessing to Abraham, God was having an account written to tell mankind who He is and all that He has done and that He wants us be in a relationship of love and trust and dependence with Him. Throughout that accounting, God interspersed little snippets of a Savior who was to come. That a day was going to come when sin would be atoned for and everlasting righteousness would come to us.
It was all a plan that God exposed to us through His written testimony to us. When Jesus died on that cross God's plan of bringing salvation to the earth was completed. It's all written down in God's testimony to us. Jesus cried out, "It is finished!" The story had been written and read and was being told one to another. The story of a Messiah to come from the people of Israel who could save us from our sin. It was finished!!! We can now be righteous before our God!!!!!
God bless,
Ted