The Catholic Church describes the Eucharist, i.e., the Sacrifice of the Mass, as “the source and summit of the Christian life” (CCC 1324). It’s the source because the Eucharist helps us to make present and offer anew Jesus Christ’s redemptive sacrifice of Calvary, which started with his Passion (CCC 1362-68; 1341). It’s the summit because the Eucharist is truly a foretaste of heaven, in which we partake of Jesus’ body and blood as heaven and earth become most profoundly one.
The Eucharist is a communion sacrifice, which means we partake of Jesus in the same way the ancient Israelites ate the flesh of the Old Covenant Passover lambs. But the New Covenant Passover is much more profound, for there is only one Lamb—the Lamb of God who takes away the sins of the world (John 1:29, 35-36)—and eating Jesus’ body and drinking his blood gives eternal life, not simply emancipation from a mere human and earthly oppressor, i.e., Egypt (John 6:52-59; see 1 Cor. 13:23-34).
Forty days after his resurrection from the dead, Jesus ascends in triumph to heaven, taking his seat at the right hand of the Father (Mark 16:19, Luke 24:50-52, Acts 1:6-11). In doing so, Jesus culminates his one sacrifice of Calvary in everlasting glory, fulfilling also the Old Covenant day of atonement/Yom Kippur sacrifices (see Lev. 16). That’s because Jesus takes not the blood of goats and calves but his own, and he enters into the heavenly sanctuary, not one made by human hands (Heb. 9:11-14). So Jesus is the high priest of heaven (Heb. 8:1-3; CCC 662-64, 1137-39), and he always lives to make intercession for us (Heb. 7:23-25; 8:1-3; 9:23-24), which implies the eucharistic sacrifice has continuing atoning power for the sins we commit daily (CCC 1366).
Jesus’ one sacrifice is made sacramentally present and offered anew at every Mass according to the order of Melchizedek, i.e., under the forms of bread and wine (Gen. 14:18-20, Heb. 5:7-10, Matt. 26:26-29, Luke 22:19-20; see CCC 1333, 1355, 1544). Subsequently, the words of the Lord’s Prayer—“Thy kingdom come, thy will be done, on earth as it is in heaven”—are nowhere more profoundly accomplished than in the Mass, because heaven and earth are united in a most perfect way that further fosters the redemption of mankind.