There is no sin so horrible that we little mortals can commit which our infinite and willing God will not forgive, provided we truly repent and seek forgiveness.
The ordinary means of God definitively extending His mercy to us in a way which we can depend on is the Sacrament of Penance.
I picked this up from the blog Good Jesuit Bad Jesuit:
Soon after his appointment, Archbishop Wojtyla approached Jesuit theologian Ignatius Rozycki and asked him to review Sister Faustina’s writings. Initially skeptical, Fr. Rozycki spent ten years in an exhaustive study of the Sister and her notebooks, which the Vatican had condemned in 1958. Father Rozycki’s findings were published and the prohibition lifted in 1978. Beatified in 1992, St. Faustina was canonized in the year 2000; on the latter occasion Pope John Paul II declared the first Sunday after Easter “Divine Mercy Sunday.”A few weeks ago I came upon a thought-provoking homily by Father Matthew Kelty, O.C.S.O., which was given on Divine Mercy Sunday, 2006. It seems that during the former Commandant’s solitary confinement in Krakow, where he awaited execution for his war crimes, Rudolph Hoess heard the bells of the local Carmel and was reminded of the Faith he had observed as a child but had long since rejected. He called for a German-speaking priest.
The local Jesuit provincial, Fr. Ladislav Lohn, S.J., went to the convent of Sister Faustina and asked the Sisters to pray earnestly while he went to hear the prisoner’s confession. In the end Hoess was reconciled with the Church and received Holy Communion. Later Hoess wrote his wife and five children, expressed sorrow for his crimes, and begged forgiveness of the people of Poland. Hoess was executed April 16, 1947.In his homily, Father Kelty contends that, though he may rightly spend an eternity in Purgatory, by the mercy of God even a man like Rudolph Hoess could be saved. This is an uncomfortable truth for some, even offensive to those whose sense of justice could be satisfied with nothing less than eternal damnation for such a “monster.”