(1) Why is missing Mass a sin, rather than just a bad idea?After all, if someone goes to daily Mass Monday through Friday, and again on Saturday morning, but decides not to go on Sunday or to the Saturday night Vigil, we're going to say he's in a state of mortal sin? Yes. And it turns out that there are a few reasons.
(2) Why do we have to attend on Sunday?
(1) Why Missing Mass is a Mortal Sin
The idea that deliberately missing Mass (and here, the Church is clear that this "deliberately" doesn't include good reasons like illness or taking care of infants - see CCC 1281) is sinful is found in Scripture, namely Hebrews 10:23-25, which says:
"Let us hold fast the confession of our hope without wavering, for He who promised is faithful; and let us consider how to stimulate one another to love and good deeds, not forsaking our own assembling together, as is the habit of some, but encouraging one another; and all the more, as you see the day drawing near."The Epistle to the Hebrews doesn't just warn against it, but provides two good reasons why it's wrong to "forsake our own assembling together":
(1) So we can "hold fast" in our own faith, andIt's easy to think of Mass as something which we do for us, that if we skip, the only people we're hurting are ourselves. But that's not true. If you've ever been to a half-hearted parish, you'll recognize the enormous importance solid communal worship has on each of our journeys towards God. It's naïve to think that our surroundings don't impact us, particularly at something communal like the Mass. We're not talking about a private act, like reading silently praying, in which you draw away from the world and your surroundings, but the Mass, which draws you into the world, your neighbors, and your surroundings. Just consider how much more enriching something like the Papal Masses are, with their throngs of Mass-goers in the hundreds of thousands, compared to an almost-empty church. So when we skip Mass, we not only deprive ourselves, but those around us, since our presence enriches their Mass experience, whether we mean it to or not. It's also at Mass that we most fully experience God, and grasp our relationship to Him. As Catholics, we're called to a personal relationship with Him, but also more than that: a corporate relationship with Him and each other, as members of the One Body He heads. Since the the Holy Sacrifice of the Mass is both the highest act of worship, and a communal act of worship, depriving ourselves of the Mass means depriving Him of the worship we owe Him in the form most pleasing to Him.
(2) So we can encourage one another.
That communal worship is what He seeks is clear to anyone who's read His meticulous plans for His Holy Temple, and carefully read the pattern in which Jesus formed concentric circles around Himself: His Holy Family, the Twelve, the other disciples sometimes referred to in the Gospel, the crowds, and then the world. With the exception of His special plans for Peter, Jesus' emphasis with His followers isn't on simply following individually and privately, like Joseph of Arimathea, but openly and together. Acts 2:42 supports this vision of what the Church is modelled to be. Given all this, intentionally thwarting God's plans for us, because we don't think we need what He's prescribing (and don't notice or care about the impact our refusal has on our neighbor) is - to put it quite bluntly - arrogant and selfish, and an offense against God and neighbor. So the Church makes clear it's an obligation. Obviously, the healthiest situation is that everyone at Mass wants to be there, and understands why their presence is importance for themselves and others, but even when we don't see why we need to go, we have an obligation to obey God. Scripture is clear on this: it's not, 'go whenever you want,' but 'do not forsake the assembly of the brethren.'
(2) Why the Obligation is Tied to Sunday
The Mass is prefigured throughout the Old Testament, most clearly in two areas: the manna and the Sabbath. The manna from Exodus 16 shows the ideal of Mass attendance. Exodus 16:4-12 says:
Then the LORD said to Moses, "I will rain down bread from heaven for you. The people are to go out each day and gather enough for that day. In this way I will test them and see whether they will follow my instructions. On the sixth day they are to prepare what they bring in, and that is to be twice as much as they gather on the other days."
So Moses and Aaron said to all the Israelites, "In the evening you will know that it was the LORD who brought you out of Egypt, and in the morning you will see the glory of the LORD, because he has heard your grumbling against him. Who are we, that you should grumble against us?" Moses also said, "You will know that it was the LORD when he gives you meat to eat in the evening and all the bread you want in the morning, because he has heard your grumbling against him. Who are we? You are not grumbling against us, but against the LORD."
Then Moses told Aaron, "Say to the entire Israelite community, 'Come before the LORD, for he has heard your grumbling.' "So the faith of the Israelites is to be tested and strengthened by daily relying upon God for sustenance. They're to receive their daily bread once per day, and twice on the day before the Sabbath. That this is a prefigurement of something is made clear from the instructions Jesus gives us for how to pray the Our Father, in which He tells us to pray, "give us this day our Daily Bread" (Matthew 6:11). So what's Christ referring to?
While Aaron was speaking to the whole Israelite community, they looked toward the desert, and there was the glory of the LORD appearing in the cloud.
The LORD said to Moses, "I have heard the grumbling of the Israelites. Tell them, 'At twilight you will eat meat, and in the morning you will be filled with bread. Then you will know that I am the LORD your God.' "
Well, the first clue is grammatical. There are at least two different Greek words for "daily" used in the Bible which are used in the New Testament, But here's the thing: the word used in Greek in the Our Father, epiousios, is found nowhere else in the Bible -- only in Matthew and Luke's accounts of the Lord's Prayer. But here's where it gets fascinating: the word used here seems to have been made up -- that is, it's not just that it's the only time epiousios is used in the New Testament, but the first time it was ever used in Greek. Like "Peter," Jesus (or the New Testament authors, depending on the language Jesus was speaking in Matthew 6) felt the need to make a new word to describe a reality apparently beyond existing language. That's a huge hint that we're not talking about ordinary bread.
The second clue is Patristics: St. Jerome translates epiousios in Luke's version of the Lord's Prayer as "daily," but when he gets to Matthew's, he translates the same word, in the same prayer, as "supersubstantial." He seems to be signalling meanings beyond simple need-fulfillment.
The third clue, of course, is Scriptural. Namely, in John 6:48-51, Jesus declares:
I am the bread of life. Your ancestors ate the manna in the desert, but they died; this is the bread that comes down from heaven so that one may eat it and not die. I am the living bread that came down from heaven; whoever eats this bread will live forever; and the bread that I will give is my flesh for the life of the world."This, He ties directly with the Eucharist, declaring in Jn. 52-58,
Jesus said to them, "Amen, amen, I say to you, unless you eat the flesh of the Son of Man and drink his blood, you do not have life within you. Whoever eats my flesh and drinks my blood has eternal life, and I will raise him on the last day. For my flesh is true food, and my blood is true drink. Whoever eats my flesh and drinks my blood remains in me and I in him. Just as the living Father sent me and I have life because of the Father, so also the one who feeds on me will have life because of me. This is the bread that came down from heaven. Unlike your ancestors who ate and still died, whoever eats this bread will live forever."So the New Manna is the Eucharist, the Body and Blood of Christ. It's suddenly clear why, in Exodus 16, the quail and manna, described by Moses and by God as the bread and meat, are tied so closely together (see Exodus 16:8,11). This is also what's meant when we hear of the blessing and breaking of the Bread thruoghout the New Testament (essentially the only food we ever hear of being blessed in Scripture, as I outlined here). So Exodus 16 prefigures the ideal for Mass-going. We should want to go daily, and if we go to the Vigil, twice on Saturday (once for Saturday, once for Sunday), just as the Israelites received physical bread daily, and twice on the day before the Sabbath (once for Friday, once for Saturday). Just as the Israelites learned from this to walk by faith in the Lord, we'll do the same, nourished by the Body and Blood of Christ.
Then there's the Sabbath, tied from the beginning with the manna: it's in Exodus 16 that God introduces the practice of keeping the Sabbath, even before Moses has received the Ten Commandments from Him ordering him to do the same (Ex. 16:23-25; Ex. 20:8). If the manna represents the ideal, the Sabbath represents the bare minimum. From the beginning, it's a commandment from the Lord (Ex. 16:23) to keep the Sabbath. Certainly, it's a commandment dispensed with for good cause, and one made for our sake (Mark 2:27), but it's still a commandment. Now, that commandment is tied to the Mosaic Law, but God makes it clear in the Ten Commandments that the Sabbath is holy from from the structure of Creation: "For in six days the LORD made the heavens and the earth, the sea, and all that is in them, but he rested on the seventh day. Therefore the LORD blessed the Sabbath day and made it holy." (Ex. 20:11; see Gen. 2:2-3). So it precedes the Mosaic Law, and survives it, but in a different form.
Just as the Saturday Sabbath celebrated God's Holiness and His Creation, so the Lord's Day, Sunday, celebrates God's Holiness through His new Creation, expressed primarily in the Resurrection (John 20:1). It's the first day of the week on the Jewish calendar, signifying the dawning light, as well as new creation (see Genesis 1:1-5; John 1:1-5; Rev 21:5). Christians quickly understood this as the focal point of Christian worship (see Acts 20:7, for example). St. Ignatius of Antioch, at the beginning of the second century, makes it clear that Christians view Sunday as a fulfillment and replacement of the Sabbath, and why, in his letter to the Magnesians:
If, therefore, those who were brought up in the ancient order of things have come to the possession of a new hope, no longer observing the Sabbath, but living in the observance of the Lord's Day, on which also our life has sprung up again by Him and by His death—whom some deny, by which mystery we have obtained faith, and therefore endure, that we may be found the disciples of Jesus Christ, our only Master—how shall we be able to live apart from Him, whose disciples the prophets themselves in the Spirit did wait for Him as their Teacher?So even by the time of Ignatius, although many Christians would attend Mass daily, Sunday was clearly set aside as a special day which needed to be observed and lived by, the focal-point and center of our lives.
To answer the question I posed earlier, about someone who attends Mass all week, but deliberately skips Sunday, that person's in grave sin for a pretty clear reason. He's not going to be saved by whatever good works going to Mass daily counts for. Rather, he'll be judged by his faith, and faithfulness - his obedience to God. If God-breathed Scripture says not to forsake the assembly of the brethren, and the assembly of the brethren meet on Sunday, you can't deliberately avoid it without sinning against God, regardless of whatever other good things you might do.