
How many of us have gone to Mass our whole lives, and know how to do all of the things at Mass, but don't really know the big picture of what we're actually doing? What about Mass is so special that the Catholic Church requires us to attend every Sunday and every Holy Day of Obligation? Why can't I just worship God in nature, in my room, or in another Christian community? What is so different about Mass that can only be done there?
Mass is not just a time to praise God--we can do that anywhere. It's not just a gathering of God's people; otherwise, I could just go to any Christian community. We don't merely listen to God's word--again, something that can be done elsewhere. Mass is the re-offering of Jesus' sacrifice on the Cross to the Father. That is why it is called "The Holy Sacrifice of the Mass." When we go to Mass, we're going to a sacrifice.
In antiquity, animal sacrifice was the norm for religious services. In the Old Testament, God commanded animal sacrifice as the proper way to worship Him. The basic idea behind animal sacrifice is returning something to God as a sign of our love, thanksgiving, atonement (making amends for sin), asking for help, etc. In animal sacrifice, the animal's pain represents the struggle through which I must go in conversion (turning away from sin and back to God).
Sacrifice comes from the Latin words sacra facere, which mean "to make holy." St. Augustine said, in City of God, “True sacrifice is offered in every act which is designed to unite us to God in a holy fellowship.” Sacrifice is an offering made to God which unites us to God and makes us holy.
Jesus instituted a new worship service--His own sacrifice. He began it at the Last Supper, turning the bread and wine into His Body & Blood. He did so separately, representing how that Body & that Blood would be separated in His passion and death the following afternoon. He definitively instituted this action, calling His Apostles to continue to offer this sacrifice, with His words: "Do this in memory of me." By this, God Himself has instituted and commanded a new form of worship. This is the proper way to worship God: offer His Body & Blood as a sacrifice to the Father on our behalf.
This is why those who lead the Masses are called "priests"--a priest is someone who offers a sacrifice. That's why Protestants have "ministers," etc. Both Martin Luther and John Calvin (the founders of most Protestant denominations) explicitly rejected the Catholic understanding of sacrifice. They incorrectly thought we were re-sacrificing Jesus at every Mass, adding more and more sacrifices to Jesus' one sacrifice on the Cross. The true understanding is that just as Jesus and the Apostles were previewing the sacrifice of Good Friday at the Last Supper on Holy Thursday, just so in every Mass afterwards, we travel back in time to the foot of the cross. At Mass, space and time are reoriented. Heaven and Earth meet, and we participate in (not merely "remember") that first Good Friday. We re-offer to God the Father the one sacrifice of Jesus on the cross.
There is, however, even more! We not only re-offer Jesus' sacrifice, but we are able to unite our own prayers (and our very selves) to the sacrifice--we spiritually place everything in our lives on the altar to be united with the bread and wine as it is transformed into Jesus' sacrifice. Our job, as the laity (the people in the pews), is to interiorly unite with the priest as he offers this sacrifice to the Father. This is how we actively participate in Mass. This is what we're doing with all of those words and gestures at Mass. This is why we have to come every Sunday and Holy Day.
At the risk of sounding like an infomercial, there's still more! Beyond offering the sacrifice as our act of worship, if we're in a state of grace, we also get to receive Jesus Himself in Holy Communion--the most intimate way to deepen one's relationship with God (the best way to become holy).
What do we offer? How do we offer it? What happens to our offering? What do we get out of all this? Find out more in my video What We Do At Mass.": https://www.youtube.com/watch?v=ddHMAKS9l3A