He was a tutor for the Radvilas (Radziwiłłs) at Biržai, a student at Oxford, headmaster of a gymnasium in Leszno, and court preacher in Königsberg and later Berlin. Of all the stages in the life of Daniel Ernst Jablonski (1660–1741), his contribution, together with Gottfried Wilhelm Leibniz, to the establishment in 1700 of the Kurfürstlich Brandenburgische Societät der Wissenschaften, the predecessor of the Prussian Academy of Sciences, is emphasised the most. However, his efforts to achieve ecumenical communication between evangelical churches of various hues were no less significant. The article deals with the development of Jablonski’s views leading to these efforts as a result of his family history: the experiences of his childhood and youth. Manifestations of efforts in East Central Europe, especially in the Commonwealth of the Two Nations, are presented through Jablonski’s activities in pursuing ecclesiastical unity, defending the rights of religious minorities, engaging in Hebrew studies, and in the ecclesiastical controversy in Russia.