The article discusses the life and scientific work of the famous poet and polemicist in 17th-century Silesia, Jan Scheffler, a sincere convert to Catholicism, who died in 1677. Jan Scheffler, poet, mystic, theological polemicist, doctor by profession, widely known under the literary pseudonym Angelus Silesius (The Silesian Angel) remained forgotten for many decades. It was only in the 19th century, and especially during the Romantic period, that attention was paid to it. A wealth of literature has been written about it. Interest in Scheffler's work has not waned even today. On the occasion of the 300th anniversary of his death, we would like to remember this figure, who left a deep mark on the minds of people at that time. Jan Scheffler was born in Wrocław in 1624. He came from a Protestant noble family, newly ennobled in Poland. He went through a breakthrough in his inner life. He came to believe that the Protestant faith was incorrect. He did not agree to the subjective and erroneous translation of the Holy Scripture by Protestants. He was offended by their rejection of tradition, abandonment of the cult of saints, and underestimation of mystical theology. He also saw a number of personal shortcomings in Martin Luther. Under the influence of these motives, on June 12, 1653, Jan Scheffler made a Catholic profession of faith in the church in Wrocław. Jan Scheffler undoubtedly presents an interesting personality and individuality. Having outstanding talents in the field of medicine, in which he was considered an expertissimus, after his spiritual breakthrough and successful conversion to Catholicism, he devoted them in poetic and polemical works to defending the Catholic Church and intensifying piety and mysticism. His scientific activity became more intense after he was ordained a priest in 1661. His zeal for the ideals of the Catholic faith is inspiring, despite the difficulties and adversities he encounters.