You can reference Mediator Dei on the Vatican web site. One of the challenges with this document is that is lacks a chapter structure. I’ll give you a reference point as we explore this 1947 encyclical letter “on the sacred liturgy.”
First off, readers should know this document was not written for lay people or parish clergy. It is addressed to bishops, and those higher up. In a way, we’ll be eavesdropping on a private conversation of the upper hierarchy just after WWII.
Comments are always welcome on these document threads, so have at it. Let’s get off to a good start. And that is Jesus Christ, the center of our Christian lives, and the center of every liturgical consideration:
Mediator between God and (people)[1 Tim. 2:5] and High Priest who has gone before us into heaven, Jesus the Son of God[Cf. Heb. 4:14] quite clearly had one aim in view when He undertook the mission of mercy which was to endow (humankind) with the rich blessings of supernatural grace. Sin had disturbed the right relationship between (people) and (their) Creator; the Son of God would restore it. The children of Adam were wretched heirs to the infection of original sin; He would bring them back to their heavenly Father, the primal source and final destiny of all things. For this reason He was not content, while He dwelt with us on earth, merely to give notice that redemption had begun, and to proclaim the long-awaited Kingdom of God, but gave Himself besides in prayer and sacrifice to the task of saving souls, even to the point of offering Himself, as He hung from the cross, a Victim unspotted unto God, to purify our conscience of dead works, to serve the living God.[Cf. Heb. 9:14] Thus happily were all (people) summoned back from the byways leading them down to ruin and disaster, to be set squarely once again upon the path that leads to God. Thanks to the shedding of the blood of the Immaculate Lamb, now each might set about the personal task of achieving his (or her) own sanctification, so rendering to God the glory due to Him.
Within the context of a post-Reformation Catholicism and in the aftermath of a tragic world war, one can understand the strong language of “wretched” and “infection.” There is a thread of optimism that in Christ, many things are possible. That last sentence about “achieving (one’s) sanctification” isn’t how I would describe the operation of grace. But in the context of liturgy, we do indeed cooperate with God’s grace, and in doing so, give God the glory.