This Sunday is called Quasimodo, from the first words of the Introit, or Dominica in Albis (post albas depositas) from the fact that on this day the newly baptized had laid aside their white vestments, or Pascha Clausum because it finishes the Easter Octave, or again, Low Sunday, perhaps in contrast to the great feast of the week before.
To teach those who, in baptism, have just been born to the life of God, the generosity with which they ought to bear testimony to Christ, the Church leads them to the station basilica of the martyr Saint Pancras, who when only twelve years old offered to Christ the testimony of his blood. Christians must stand firm, resting on their faith in Christ, the risen Son of God. St. John tells us that this is the faith that overcomes the world, for it enables us to resist all efforts to make us fall (Epistle).
Thus it is important that it should have a firm foundation, which the Church gives us in today's Mass. St. John says in the Epistle that this faith is founded upon the witness of the Father, who at our Lord's baptism (with water) proclaimed Him His Son; of the Son who on the cross (by His blood) showed Himself as the Son of God; and of the Holy Ghost, descending on the Apostles on the day of Pentecost, according to our Lord's promise, confirmed what Christ had said about His resurrection and His divinity, dogmas which the Church, guided by the Holy Ghost, never ceases to proclaim.
Our faith rests also on the testimony of angels who announced our Lord's rising from the dead (Offertory), but it is based chiefly on His appearances to His apostles. Further, the Gospel shows us how Christ, appearing twice in the Cenacle, overcame the unbelief of St. Thomas, praising those who, not having seen, should yet believe.