The paper deals with the international activities of the Holy See, in the context of the mission of the Church, with special emphasis on the concordat law and its concretization in the treaties between the Holy See and the Republic of Croatia. It starts from the basic premise of the separation of Church and State, which enables freedom to fulfil the mission of the Church. Then, it elaborates the historical-theological principles from Augustine to the Concordat of Worm presenting the experience of the Church in its relation to state power. It then observes the Holy See within the international community of states in a historical, legal and diplomatic context, showing its status as a sovereign, recognized, equal and active subject of international relations. It points out the mission of the Church as the motive and foundation for the international activity of the Holy See, which, especially today, opens the space of exceptional evangelizing potential. It ends with the elaboration of the concordat law of the Holy See, showing the process of passing international treaties and the place of concordats in contractual international law. To concretize the above, it analyses the realization of the work of the Church through the international activities of the Holy See on the example of the Treaties between the Holy See and the Republic of Croatia. The mission of the Church in the Croatian Treaties is realized in the field of pastoral care, education, health, charitable activities, etc., in fact regulating the framework in which the activities of the Church are found in every aspect of society.