Only God is capable of perfection. Only He can do something that is in every sense complete.
When something is Finished there is nothing else to be done. All that is needed is provided.
As Jesus gave up His Spirit to the Father He said " Finished !". This statement is transcendental. It is more profound than we can fully grasp. The totality of it, the universality of it, the limitlessness of it is to be explored but never fathomed. It is Grace.