The Second Vatican Council recognised the principle of cooperation between the two communities as one of the foundations to develop relations between the Church and the State. Before its final proclamation under the number 76 of the Pastoral Constitution on the Church in the Modem World Gaudium et spes, the Council Fathers had thoroughly analysed the existing models and foundations of the relationship of the Church and the State. In this article one attempted to present the Council's path to a deeper understanding of the Catholic ecclesiology in mutual relations of the community of the Church and of the State, the postulate of which is the principle of cooperation. The proposals that were subject to discussion at the general congregations during the preparatory period of the particular document schemes were not always approved. The participants of the Council once again read the Church's mission in the world, that is why they referred so critically to the so-called “unfortunate separation" (infausta separatio) of Church and State. They decided that those relations should be dictated by sincere cooperation of the two communities and their authorities, as the same people belong both to the Church and the State; only the reasons for their affiliations vary.