Faustina wrote a diary. She did this despite the fact that she could hardly write at all. The diary was later published under the title Divine Mercy in My Soul: The Diary of St. Faustina. Faustina unsuccessfully tried to found a “Congregation which will proclaim the Mercy of God to the world, and, by its prayers, obtain it for the world”, but was constantly denied leave by her convent.

By June of 1938, she could no longer write in the diary, and it became obvious that she would not live much longer. St. Faustina died on October 5. The task of spreading the message of Divine Mercy, already well begun, was continued by her spiritual director, Father Sopoćko. Faustina had never been able to found the religious order which Jesus had asked for, but she had left clear rules for the life of the prospective community, and at last in 1941, the order, now known as the Institute of Divine Mercy, was founded.