Josip Juraj Strossmayer 's ideas , of which the most important one was - serving God equals to serving people , created close relations between Croats and Serbs by introducing the Old Slavonic language as the liturgy language of the Roman Catholic Church in the Balkans - were aggressively suppressed by the Roman Catholic clergy in Croatia and Slovenia . Josip Juraj Strossmayer 's ideas , of which the most important one was that serving God is equal to serving the people , created close relations between Croats and Serbs by introducing the Old Slavonic language as the liturgical language of the Roman Catholic Church in the Balkans and were aggressively suppressed by the Roman Catholic clergy in Croatia and Slovenia .