The words “working in us that which is pleasing in his sight” describe what happens when God writes the law on our hearts in accord with the new covenant. And the words “through Jesus Christ” describe Jesus as the mediator of this glorious work of sovereign grace.
So the meaning of Christmas is not only that God replaces shadows with reality, but also that he takes the reality and makes it real to his people. He writes it on our hearts. He does not lay his Christmas gift of salvation and transformation under the tree, so to speak, for you to pick up in your own strength. He picks it up and puts it in your heart and in your mind and gives you the seal of assurance that you are a child of God.