It's the vessel for drawing graces.
Why is trust in Jesus so important in understanding Divine Mercy? Because it is our response to God's great mercy. Our part is to trust in Him. In fact, the image of The Divine Mercy bears the inscription "Jesus, I trust in You!" inviting us to answer this call.
In her Diary entry 1578, St. Faustina records our Lord's revelation:
The graces of My mercy are drawn by means of one vessel only, and that is — trust. The more a soul trusts, the more it will receive. Souls that trust boundlessly are a great comfort to Me, because I pour all the treasures of My graces into them. I rejoice that they ask for much, because it is My desire to give much, very much. On the other hand, I am sad when souls ask for little, when they narrow their hearts.
What is trust? Trust is our faith, hope, and love in action. Trust is an action verb that takes in all three. It combines the past focus of our faith in what Jesus did, the present "now" dimension of His love for us, and the future dimension of hope because of what He has prepared for us in heaven.
Trust, then, means to believe in Jesus, to love Him, and to hope in Him. It means to be totally absorbed in Jesus as our Lord and Savior, to rely completely upon Him. We desire to have His mind and thoughts, His will, His power, His Heart, and His total trust in the Father. To sum up, when we really trust in Jesus, we can say with the Apostle Paul: "It is no longer I that live but Christ who lives in me" (Gal 2:20).
But how do I trust? The Blessed Virgin Mary is our great example here as the model disciple of the Lord. We see her at the Annunciation completely abandoning herself to God's will when she was asked to be the Mother of the Son of God.
The eminent Polish theologian Fr. Ignacy Rozycki produced a brilliant essay on the Diary entitled "Essential Features of the Devotion to The Divine Mercy." In the section of his essay on Divine Mercy Sunday, he explained that one's focus should be on receiving the sacraments worthily and completely trusting in Jesus. Through this condition, it's as if the slate is wiped clean, and one is given a completely fresh start in life. He wrote, "All graces and benefits, even in their highest degrees, are accessible on this day to everyone, as long as they are asked for with great trust."
Read more about trust.