If we are currently in the New and Everlasting Covenant (New Testament), then why do we use Old Testament readings for our First Reading at Mass?
Some Catholics presume that the four Gospels form the heart of the Catholic Bible, but the Old Testament holds an important place as well, especially in the Mass.
The Church’s Liturgy makes extensive use of Scripture. Every liturgical rite (the Mass and the other
sacraments, the Liturgy of the Hours, and other rites) include the proclamation of biblical readings, especially from the Gospels, and often from the Old Testament. The liturgical texts themselves draw from the Scriptures, too. Particular images, sayings, and expressions are often quoted directly from biblical texts. The Book of Psalms, the first “prayer book” of the Church, has always been a source of the language of liturgical prayer.
In the celebration of Mass on Sundays and Solemnities, three readings from Scripture are proclaimed (not counting the Psalm, which, although normally sung in part by the liturgical assembly, is a part of the proclaimed Word of God): a reading from the Old Testament (except during the Easter Season, when the first reading is taken from the Acts of the Apostles), a reading from one of the New Testament letters or the Book of Revelation, and a Gospel reading. On weekdays, two readings are used, selected in “semicontinuous” fashion (that is the use of large excerpts continuing from one day to the next): the first is either from the Old Testament or the New Testament, and the second is a Gospel reading.
The Introduction to the Lectionary for Mass explains that the guiding principle for the selection of the readings on Sundays and feast days the principle of “harmony.” It calls for texts that complement one another thematically, usually centered on the gospel reading. Readings are chosen from the Old Testament that anticipate or reflect the event or theme of the Gospel reading or the feast. For example, on Christmas Eve, the Gospel reading, Luke 2:1–14, tells the story of the birth of Jesus in the stable at Bethlehem. The first reading, from the Prophet Isaiah (9:1–6), announces the birth of the child who is named “Wonder–Counselor, God–Hero, Father–Forever, Prince of Peace.” The harmony of these texts demonstrates what Catholics believe about the Bible, about Jesus, and the relationship between the Old and New Testaments. The context of salvation history for Christians is Jesus, and the Old Testament, as it depicts the unfolding of God’s creation, covenant, and relationship with his people, prepares for and leads to the coming of Christ in human history in his Incarnation in the flesh.
(Source: United States Council of Catholic Bishops)
Read more in the “Why Do We Do That?” series from Deacon Mike Fritz.