Quote: CCC # 2182: Participation in the communal celebration of the Sunday Eucharist is a testimony of belonging and of being faithful to Christ and to his Church. The faithful give witness by this to their communion in faith and charity. Together they testify to God's holiness and their hope of salvation. They strengthen one another under the guidance of the Holy Spirit.
As you know, Christians transfer the observance of the Sabbath from Saturday to Sunday (i.e., the Lord's Day; as described in CCC #2174 & 2175). The Catechism teaches that attending Sunday Mass is one of the ways in which we obey this commandment:
Simply put, if people believe that Sunday Mass is no big deal, then such a view is contrary to the beliefs of the martyrs, to what has historically been taught in Christianity, and to the biblical passages I quoted above. On the other hand, those who believe that Sunday Mass is, indeed, a big deal must admit that that skipping it is likewise a big deal (or else there is a strange disconnect between the great worth of the Mass that one professes, and the small value one ascribes to it when it is missed). In stating that to knowingly and willingly miss Mass on a Sunday or Holy Day of Obligation results in mortal sin, the consequence of rejecting so great a gift is clearly depicted. Participating in Sunday Mass is a life-giving experience, and the only realistic consequence of a serious rejection of life-giving grace is spiritual death, which culpability to mortal sin entails.