Sunday is a “day of faith,” according to John Paul II. Note the distinctive qualities of Sunday Mass above liturgy celebrated on the other six days, he advises.
29. Given these different dimensions which set it apart, Sunday appears as the supreme day of faith. It is the day when, by the power of the Holy Spirit, who is the Church’s living “memory” (cf. Jn 14:26), the first appearance of the Risen Lord becomes an event renewed in the “today” of each of Christ’s disciples. Gathered in his presence in the Sunday assembly, believers sense themselves called like the Apostle Thomas: “Put your finger here, and see my hands. Put out your hand, and place it in my side. Doubt no longer, but believe” (Jn 20:27). Yes, Sunday is the day of faith. This is stressed by the fact that the Sunday Eucharistic liturgy, like the liturgy of other solemnities, includes the Profession of Faith. Recited or sung, the Creed declares the baptismal and Paschal character of Sunday, making it the day on which in a special way the baptized renew their adherence to Christ and his Gospel in a rekindled awareness of their baptismal promises. Listening to the word and receiving the Body of the Lord, the baptized contemplate the Risen Jesus present in the “holy signs” and confess with the Apostle Thomas: “My Lord and my God!” (Jn 20:28).
The Creed, recited or sung, yes–this is important. Does faith sink deeper for us? Does Sunday inspire a heartfelt acclamation from within, as the apostle uttered. The encounter cited here actually isn’t so much about faith as it is about a movement. Thomas lacked belief on Easter Sunday: this is true. But he was willing to change, to reform, and to be converted. When presented with the signs, he uttered one of the most basic formulas of belief in Christ: My Lord and my God.