These letters speak to the whole Church about faith, holiness, love, patience, discernment, and remaining faithful to Christ.
Why it comes after the Pauline reading
The sequence of readings lets the Church hear apostolic teaching from more than one witness. St. Paul is read, then the general apostolic letters are read, then the Acts of the Apostles, then the Synaxarium, Psalm, and Gospel. The order teaches that the faith is apostolic, shared, and received by the whole Church.
Be mindful of the words which were spoken before by the holy prophets, and of the commandment of us, the apostles of the Lord and Savior.
What to listen for around the Catholic Epistle
Listen for practical Christian instruction. The Catholic Epistle often speaks directly about how faith becomes patience, mercy, purity, obedience, watchfulness, and love. It reminds the faithful that doctrine becomes visible in conduct.
This reading also keeps the Liturgy from narrowing apostolic teaching to only one voice. The Church hears St. Paul, then hears the broader apostolic witness, then hears Acts and the Gospel. By the time the Creed is confessed, the people have heard the faith from Scripture in several forms: teaching, exhortation, history, psalmody, and the words of Christ.