Saint Maria Faustyna Kowalska of the Blessed Sacrament, OLM, popularly spelled Faustina (born as Helena Kowalska), was a Polish Roman Catholic nun and mystic. Her claims of receiving apparitions of Jesus Christ inspired the Roman Catholic devotion to the Divine Mercy and earned her the title of "Apostle of Divine Mercy".

Throughout her life, Faustina reported having visions of Jesus and conversations with him, of which she wrote in her diary, later published as The Diary of Saint Maria Faustina Kowalska: Divine Mercy in My Soul. Her biography submitted to the Congregation for the Causes of Saints quoted some of these conversations with Jesus regarding the Divine Mercy devotion.

At the age of 20 years she joined a convent in Warsaw, Poland, was later transferred to Płock, and then to Vilnius where she met her confessor Father Michał Sopoćko, who supported her devotion to the Divine Mercy. Faustina and Sopoćko directed an artist to paint the first Divine Mercy image, based on Faustina's vision of Jesus. Sopoćko used the image in celebrating the first Mass on the first Sunday after Easter. Subsequently, Pope John Paul II established the Feast of Divine Mercy on that Sunday of each liturgical year.

The Roman Catholic Church canonized Faustina as a saint on 30 April 2000, considering her a virgin and mystic. She is venerated within the Church as the "Apostle of Divine Mercy".

A great biography of the Saint Faustina:
https://www.goodreads.com/book/show/2...