The Incredulity of St. Thomas, by Carravaggio (+1610AD)
This Sunday, the Octave day of Easter, probably has more names and designations than any other day in the Church calendar: the Octave of Easter, the Second Sunday of Easter, Quasimodo Sunday (from the Introit of the day), Dominica in Albis (recalling the white vestments of the newly Baptized), Low Sunday (compared to the high of Easter!), Thomas Sunday (recalling the Gospel), and, of course, Divine Mercy Sunday.
For more on Easter, in general, you should note:
Old Catholic Encyclopedia: Easter
Fish Eaters: Eastertide
The Introit for today is the origin of one of these names, and here is a recording of the same:
Today's Gospel, from that of St. John, reads:
"Chapter 20:19 And now it was evening on the same day, the first day of the week; for fear of the Jews, the disciples had locked the doors of the room in which they had assembled; and Jesus came, and stood there in their midst; Peace be upon you, he said. 20 And with that, he shewed them his hands and his side. Thus the disciples saw the Lord, and were glad. 21 Once more Jesus said to them, Peace be upon you; I came upon an errand from my Father, and now I am sending you out in my turn. 22 With that, he breathed on them, and said to them, Receive the Holy Spirit; 23 when you forgive men’s sins, they are forgiven, when you hold them bound, they are held bound.
24 There was one of the twelve, Thomas, who is also called Didymus, who was not with them when Jesus came. 25 And when the other disciples told him, We have seen the Lord, he said to them, Until I have seen the mark of the nails on his hands, until I have put my finger into the mark of the nails, and put my hand into his side, you will never make me believe. 26 So, eight days afterwards, once more the disciples were within, and Thomas was with them; and the doors were locked. Jesus came and stood there in their midst; Peace be upon you, he said. 27 Then he said to Thomas, Let me have thy finger; see, here are my hands. Let me have thy hand; put it into my side. Cease thy doubting, and believe. 28 Thomas answered, Thou art my Lord and my God. 29 And Jesus said to him, Thou hast learned to believe, Thomas, because thou hast seen me. Blessed are those who have not seen, and yet have learned to believe.
30 There are many other miracles Jesus did in the presence of his disciples, which are not written down in this book; 31 so much has been written down, that you may learn to believe Jesus is the Christ, the Son of God, and so believing find life through his name."
Hence, this day recalls the roll of St. Thomas -- the Apostle that doubted. I prefer to recall St. Thomas having earlier noted in the Gospel of St. John: "11:16 Thereupon Thomas, who is also called Didymus, said to his fellow-disciples, Let us go too, and be killed along with him," and interpret his incredulity in this light: his love for Christ was such that it would not admit to allowing a hope that was not well-founded. All the same, it is he that proclaims: "Thou art my Lord and my God."
Today is also, more recently, known as Divine Mercy Sunday. This was a particular devotion and focus on the mercy of Almighty God made popular by the Polish Saint, St. Faustina, and approved by Pope Blessed John Paul II.
The Vatican promulgated a decree regarding the wonderful Indulgence available, under the usual conditions, on Divine Mercy Sunday:
Vatican Decree on Indulgence attached to devotion to Divine Mercy
For more on the devotion to the Divine Mercy, you should note:
EWTN: Divine Mercy Novena
Today, perhaps, we might review the Bull of Indiction of the Extraordinary Jubilee of Mercy promulgated by Pope Francis for this year of Mercy. You can find the full text here:
Misericordiae Vultus: BULL OF INDICTION OF THE EXTRAORDINARY JUBILEE OF MERCY
Surrexit Dominus vere, alleluia, et apparuit Simoni, alleluia!