This article presents connections between the Polish - Ukrainian dialogue and reconciliation with historical forms of ecumenical aspirations, which arose on the Ruthenian territory of the First Republic of Poland. It is a look at the past which seeks inspiration and arguments in support of the inclusion of the Orthodox Church into the Polish-Ukrainian dialogue. This dialogue should be this way balanced and broadened. Because so far, it has been mainly led within the Catholic Church of the Latin rite and the Byzantine-Ukrainian Church. This article points out several examples of how and where the ecumenical potential of the Polish-Ukrainian dialogue can be spotted. These examples are connected with:  universalist understanding of the sacrament of baptism on the Polish-Ruthenian borderland in the Middle Ages,  ecumenical thought of the Reverend Stanislaw Orzechowski,  teaching about sacraments of baptism and marriage administered in the Catholic Church and the Orthodox Church of Piotr Mohyla.