Yes, Skipping Sunday Mass (Without a Good Reason ) Is a Grave Sin
Of the roughly 68 million people who identify as Catholic in the United States, only about a third go to Mass on any given weekend. This is obviously a big problem!
It's a problem for a number of reasons, not least of which that Mass is a source of necessary grace for our souls. But there's another serious reason that many self-identified Catholics may not fully understand: If a Catholic knows that Mass attendance is required on Sundays and Holy Days of Obligation, has Mass nearby he can attend, does not have a "grave cause" to miss Mass, but still skips Mass, he commits a grave sin.
Yes, you aren't just a bad Catholic, you're committing a sin you will need to confess in Reconciliation for the sake of your soul.
Mass isn't just a nice thing to do if you're particularly religious or "super Catholic"; rather, it's the cornerstone of any authentic Catholic life. In fact, of the five precepts of the Church (what the Catechism calls the "necessary minimum" for being a practicing Catholic), attending Mass on Sundays and Holy Days of Obligation is the first one! This requirement is also made explicit in the Code of Canon Law (CIC 1247Jf).
Of course, as mentioned above, it's not a sin to miss Mass if you have a good reason; for example, if you are sick or have to care for others. If you are in that situation, know that you are still an integral part of the Church, that the Church prays for you, and that the Church covets any prayers you can offer.
What if you are one of the millions of Catholics who regularly skips Mass without a good reason? What should you do? The first thing you should know is that the Church still wants you! You can always come back - and you should for the sake of your soul.
If you are ready to re-commit to the Catholic life, including weekly Mass attendance, you should go to Confession to get a fresh start. Remember, if we repent, Jesus is always willing to receive us back.
So we encourage all Catholics: go to M ass!