Recognize the Eucharistic Jesus and receive His gifts.
As recorded in the Diary of St. Faustina, our Lord promises extraordinary graces to souls who will go to confession (before Divine Mercy Sunday) and worthily receive Holy Communion on this Feast or its Vigil.
Through this promise, our Lord is emphasizing the infinite value of confession and Communion as miracles of mercy. He wants us to realize that since the Eucharist is His own Body, Blood, Soul, and Divinity, it is the "Fountain of Life" (Diary, 300). The Eucharist is Jesus, Himself, the Living God, longing to pour Himself as Mercy into our hearts.
Why would our Lord feel the need to emphasize this? Because so many people do not really understand it. They either see no need to receive Holy Communion, or they receive it simply out of habit. As St. Paul explains in his letter to the Corinthians, they eat the bread or drink the cup of the Lord unworthily, "without recognizing the body of the Lord" (1 Cor 11:27-29).
In His revelations to St. Faustina, our Lord makes it clear what He is offering us in Holy Communion and how much it hurts Him when we treat His presence with indifference:
My great delight is to unite Myself with souls. … [W]hen I come to a human heart in Holy Communion, My hands are full of all kinds of graces which I want to give to the soul. But souls do not even pay any attention to Me; they leave Me to Myself and busy themselves with other things. Oh, how sad I am that souls do not recognize Love! They treat Me as a dead object.
Diary, 385; also see 1288 and 1447
So, our Lord's promise of complete forgiveness is both a reminder and a call. It is a reminder that He is truly present and truly alive in the Eucharist, filled with love for us and waiting for us to turn to Him with trust. And it is a call for us all to be washed clean in His love through confession and Holy Communion — no matter how terrible our sins — and begin our lives again. He is offering us a new start.
To read more about Communion on this feast, download Understanding Divine Mercy Sunday.