From the time of Charlemagne, the German Church was regarded as the only one entitled to carry out missionary and ecclesiastical activities in Central and Eastern Europe. However, its missionary activity made the Christianised peoples politically dependent on the Germans and imposed German mediation between the peoples won to the faith and the See of Peter. Through their missionary activity, the first Piasts demonstrated a full understanding of the principle that one of the main characteristics of an independent Christian ruler was to spread and strengthen the faith not only in the lands directly under his control, but also beyond their borders. The subject of this article is an attempt to present the forms of missionary activity carried out between 985 and 1025 by Saints Adalbert, Bruno and Swierad in the Slavonic lands on both sides of the Carpathians.