"Where sin increased, grace increased all the more" writes St. Paul in Romans 5:20 and it does not take a lot of imagination to see the first part of that sentence realised horribly in the 20th Century. But if one believes that Paul's words came from the Holy Spirit, the remaining part of the verse must also hold true. The horrors of WW2 cannot be erased or dealt with, but God has ensured that His children are not left in total despair without a way out. And, as usually, He did that through the hands of a single humble person. Sister Maria Faustyna Kowalska was up for the task.

St. Faustina, as she is known today, was supposed to bring the message about God's mercy to the whole world. It was not meant to change or add to the way we understand God or anything about the Revelation, but to give new power to one of His attributes that might have been put in question by the upcoming events. Faustina wrote, and later rewrite, her diary inspired by her spiritual directors and remained faithful to the precept in spite of numerous doubts. She started in 1934 and finished in 1938, three months before her death, a year before WW2 broke out.

Apart from documenting daily events from conventual life, with common struggles of relations with other nuns, battling weaknesses and deficiencies of human nature, the Diary contains a detailed record of Faustina's spiritual life, her mystical communion with God, and most of all, the words of Jesus directed to her and the whole world. It is the antecedant of the image of Merciful Jesus, better known as "Jesus, I trust in you", the Divine Mercy Chaplet and Novena, and other ways of devotion to Divine Mercy.

The goal has been achieved: the message spread to even the most remote ends of the Catholic Church. It fueled the faith of today's Catholics with new hope and love, and let the Church prevail. With that, "Diary: Divine Mercy in My Soul" became the most popular and widely translated book by a Polish author with some claiming that it is the second most important book in the world.