ABSTRACT: Aim: The purpose of the article is to present the life of Saint Sister Faustina Kowalska with particular attention on her loneliness in God. Methods: Analysis of dozen available biographies of Saint Faustina Kowalska and the „Diary” written by her. Results: Sister Faustina was gifted by God with many extraordinary graces, she experienced revelations, ecstasies, the gift of bilocation, great suffering, reading in human souls, mystical engagements and weddings. She was basically very lonely throughout her life, but it didn't bother her because God was always with her. This simple girl, who had only completed three grades of primary school, completely surrendered herself to God's will and, despite many problems she encountered on her way, fulfilled every task given to her by the Lord Jesus. Outside, Sister Faustina's life was very ordinary, every day was basically the same. She was a gardener, a cook, and in the last year of her life, as much as her health allowed, she served as a porter. Everything she did was dedicated completely to the Lord Jesus. Conclusion: The most important task of Saint Sister Faustina Kowalska was to convey the message of Mercy to the Church and the world, which is a reminder of the biblical truth about God's merciful love for every person, a call to entrust one's life to Him and actively love one's neighbors. It was the Savior who gave her the "vessels" thanks to which we can draw the graces of Divine Mercy. Sister Faustina is the author of the famous „Diary”, a bestseller translated into over 30 languages. By getting to know her notes, we see what a strong bond she had with the Lord Jesus. Today Saint Sister Faustina is one of the most popular saints of our time. Pilgrims from all continents make pilgrimages to her relics located in the Sanctuary of Divine Mercy in Kraków-Łagiewniki. Originality: The article is the quintessence and at the same time a pleasant presentation of the life of Saint Faustina Kowalska, resulting from the review and analysis of several available studies and biographies of her life. Keywords: Helena Kowalska, Sister Faustina, Secretary of Merciful Jesus, Apostle of Divine Mercy, Prophet of our times, Great Mystic, Mistress of spiritual life, Feast of Divine Mercy, Chaplet, Hour of Mercy.