In different convents throughout Poland she performed very ordinary duties - cook, gardener and doorkeeper and suffered from poor health.  Externally nothing revealed the rich prayer life she had with Jesus and the mystical graces he gave her. She developed a child-like trust in God as well as mercy toward her neighbors.  In her spiritual life she showed a love of the Eucharist and a deep devotion to the Mother of Mercy.

On February 22, 1931, Jesus appeared to this simple nun:

In the evening, when I was in my cell, I became aware of the Lord Jesus clothed in a white garment. One hand was raised in blessing, the other was touching the garment at the breast. From the opening of the garment at the breast there came forth two large rays, one red and the other pale. In silence I gazed intently at the Lord; my soul was overwhelmed with fear, but also with great joy. After a while Jesus said to me: “Paint an image according to the pattern you see, with the inscription: Jesus, I trust in You”.

Jesus asked her to sacrifice her life to make God’s mercy known to every human being and  entrusted her with a mission of mercy.  The mission consisted of three tasks:

1. To remind the world of God’s merciful love of God for every human being, no matter how great their sins.
2. To pray for God's mercy for the whole world and particularly for sinners, especially through the practice of new forms of devotion to the Divine Mercy presented by Jesus, such as: the veneration of the image of the Divine Mercy with the inscription: Jesus, I Trust in You, the feast of the Divine Mercy celebrated on the first Sunday after Easter, the chaplet to the Divine Mercy and prayer at the Hour of Mercy (3 p.m.). Jesus attached great promises to these forms of devotion, provided the person entrusted their life to God and practised active love of neighbour.
3. To initiate an apostolic movement of Divine Mercy. This involved proclaiming and entreating God's mercy for the world and striving for Christian perfection after the manner of Sister Faustina, especially in child-like trust in God and mercy toward one's neighbours.

The movement has spread throughout the Church and  involved millions of people: religious congregations, lay institutes, religious, brotherhoods, associations,  communities of apostles of the Divine Mercy, as well as individual people who take up the tasks Jesus communicated to them through Sr. Faustina.

Faustina's life in the convent was filled with extraordinary gifts: revelations, visions, hidden stigmata, sharing in the passion of Jesus, bilocation, the reading of human souls, the gift of prophecy, and the rare gift of mystical engagement and marriage.  These and her mission are all recorded in her Diary, which she kept faithfully in obedience to Jesus and her spiritual director. 

Jesus chose Sr. Faustina as the "Secretary" of his mercy, so that she could tell the world about his great message. He said to her:

In the Old Covenant I sent prophets wielding thunderbolts to my people. Today I am sending you with my mercy to the people of the whole world. I do not want to punish aching mankind, but I desire to heal it, pressing it to my merciful heart (Diary 1588).

She recalls how Jesus told her:

Secretary of my most profound mystery,  know that your task is to write down everything that I make known to you about my mercy, for the benefit of those who by reading these things will be comforted in their souls and will have the courage to approach me (Diary 1693).

In an extraordinary way, Sr. Faustina's work sheds light on the mystery of the God's mercy. It delights not only the simple and uneducated people, but also scholars who see it as an additional source of theological research. The Diary has been translated into many languages - English, German, Italian, Spanish, French, Portuguese, Arabic, Russian, Hungarian, Czech and Slovak.

Weakened by tuberculosis and sufferings which she accepted as a voluntary sacrifice for sinners, she died in Krakow in 1938 at the age of just 33.

Sister Faustina was beatified by Pope John Paul II on the 2nd Sunday of Easter in 1993 and canonised on the 2nd Sunday of Easter in 2000.  In an extraordinary coincidence the Gospel of this Sunday (John 20:19-31) in the Lectionary as revised after Vatican II is the Gospel of Divine Mercy.  The Risen Jesus breathes on his disciples, saying: “Receive the Holy Spirit. For those whose sins you forgive, they are forgiven; for those whose sins you retain, they are retained”.