Have you ever been to a wedding? There are a lot of people in the wedding. There is flower girl, bridesmaids, groomsmen, ushers, wedding coordinator, etc. All of these people add to the celebration of the wedding.
However, if they don't come, the wedding would still happen. The only two people needed for a wedding are the bridegroom and the bride. If one of them does not show up, the wedding doesn't happen.
The Bible says that Jesus is the bridegroom and the Church is His bride (Song of Songs, John 3, Revelation). At Mass, we are called to the wedding feast of Heaven where we are united to our bridegroom in the Holy Sacrifice of the Mass. We can always count on the bridegroom showing up.
When we are at Mass, we renew our Covenant with Him … the New Covenant that he instituted with us at the Last Supper. It is this covenant … this promise made by the one who loves us … that allows us to one day spend eternity with our bridegroom in Heaven.
This covenant is something that has to be renewed every week on the Sabbath. Genesis 1 says that God created in six days and on the seventh day he rested. The number seven in Hebrew is 'sheba' which is also the Hebrew word for covenant.
On the seventh day in the book of Genesis, God created a covenant with man and the seventh day would be put aside for creation to worship her Creator. This is also why God commands us to worship Him, to “keep holy the sabbath day” in the ten commandments (Exodus 20:8-11).
When we skip Mass on Sunday, we are violating our covenant with God. We are saying to God, 'I don't need to be united to you. I don't need to worship you.' It may seem innocent, but we are actually declining His marriage proposal. We are not showing up for our own wedding.
When we make a decision to deny His invitation to covenant we are saying we don't want a relationship with Him. Because our God loves us and is a gentleman, he allows us to do this.
However, it’s a grave sin to miss Mass.
If we have full knowledge that missing Mass is a grave sin and we fully consent to it (i.e. being sick or getting in a car accident on the way to Mass would be legitimate excuses for missing Mass), then we are in a state of mortal sin … we have cut off our relationship with God and need to seek the Sacrament of Reconciliation before we receive Holy Communion again.