Since their arrival in Poland in 1564, the Jesuits have developed a wide range of activities. They saw themselves as leaders of the Counter-Reformation and worked hard to raise the spirit of the Catholic Church as it struggled against the rising tides of Protestantism. One of their priorities, apart from organizing trips to preach the Gospel and establishing universities and seminaries, was publishing. In numerous publications, the Jesuits exposed the errors of dissenters, defended Catholic doctrine and exhorted the faithful not to neglect their spiritual life. The most extraordinary of the early works of the Polish Jesuits was undoubtedly "The Spiritual Harp" by Marcin Laterna. Published in Kraków in 1585, it had several dozen reprints in the following years. Thanks to its extraordinary popularity, which lasted almost three centuries, "The Spiritual Harp" played an important role in shaping the piety of the Catholic masses and their understanding of Church doctrine. An exposition of the latter is found in the first chapter entitled "Summary of Christian Doctrine", in the general introductions to the individual chapters, and in the notes accompanying the most important prayers. In his lecture on Catholic teaching, the author refers to the Holy Scripture, the Catholic tradition (represented, among others, by the Doctors of the Church and medieval authors) and the declarations and explanations of the Council of Trent (1545-1563). The selection of prayers indicates a preference for Roman missals and breviaries, although the old "Hortuli" are also used. The intention to inculcate in the faithful the habits of spiritual life is clearly visible in the reflections on some Bible verses and in the insistence with which the author mentions examination of conscience and participation in the Holy Mass. The influence of Laterna's work extended beyond its primary users, and provided a model for many other prayer book authors. It is hoped that the extraordinary richness of "The Spirit's Harp" will be the subject of further research.