Assurance of victory comes as a response to prayer. Jesus, who can’t lie nor be mistaken, had said this: “Everyone who asks always receives; the one who searches always finds; the one who knocks will always have the door opened to him” (Mt. 7:8).
That is the assurance the Divine Mercy devotion gives us, especially for the 3 o’clock prayer and the recitation of the chaplet of Divine Mercy. Talking about the chaplet which St. Faustina was praying on an occasion when the heat was so intense and the earth needed rain, she had been saying it without interruption for three hours, then the heavy rain came. She comments on this chaplet: “And the Lord let me know that everything can be obtained by means of this prayer” (paragraph 1128 of her book – Diary of Divine Mercy in my Soul). The Lord spoke to her about the 3 o’clock prayer. “At three o’clock, implore My mercy, especially for sinners; and, if only for a brief moment, immerse yourself in My Passion, particularly in My abandonment at the moment of agony. This is the hour of great mercy for the whole world. I will allow you to enter into my mortal sorrow. In this hour, I will refuse nothing to the soul that makes a request of Me in virtue of My Passion….” (paragraph 1320, with the same book, Diary of Divine Mercy in my Soul, by St. Maria Faustina Kowalska).
St. Alphonsus Liguori said that one of the biggest regrets people will have if they are punished in eternal Hell is why they did not ask or pray when the Lord had clearly said everyone who asks always receives (Mt. 7:8). Yes, I mean those who ask with faith, sincerity, perseverance, and humility.