(This is a little bit I wrote as a comment to a post of my dear blogfriend JessicaHof, “Why go to Church?” The discussion there is worth reading, but I also thought my response here might make a rare, short, succinct post for The Lonely Pilgrim.)
The deeper Catholic reason why Catholics must go to Mass, and why it’s a mortal sin to miss it, is because we are members of the Body of Christ; we are the Church. The Eucharist is the source and summit not only of our faith, but of our lives. To abandon the Body is to abandon ourselves, and to neglect the Body and Blood is to neglect our own spiritual and even physical well-being.
Many Protestants, especially evangelical, “Bible” Protestants, have it in their heads that they can be Christians just by being alone with God and with their Bibles. Such a thought is incomprehensible to a Catholic; because we are members of a Body. How we worship God, how we receive His graces, is what we do together, in communion with each other and with Christ. We go to church because it is only in our corporate worship that we come face to face, flesh to flesh with Jesus.
Before I became Catholic, I wrote a lot to myself and made a lot of lists trying to establish the reason for going to church, and therefore what I was really looking for in a church. I decided that the primary purpose of the church was to “support and nourish the body of believers” — I used that very word, nourish. What I had in mind was fellowship and a system of support, to hold up believers and see to their well-being; to see they are being fed (I used those words, too) with the Word of God. When I first came to the Catholic Church, I didn’t immediately find that fellowship I was looking for — but I found what I really needed, what my soul cried out for: communion with Christ and with His Body on earth, being nourished and fed by His very Body and Blood.