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.