The Ruthenian Greek Catholic Church was created with the Union of Brest in 1595/1596, yet its roots go back to the very beginning of Christianity in the Mediaeval Slavic state of Ruthenia. Byzantine missionaries exercised decisive influence in the area. The 9th-century mission of Saints Cyril and Methodius in Great Moravia had particular importance as their work allowed the spread of worship in the Old Church Slavonic language. The Byzantine-Greek influence continued, particularly with the official adoption of Byzantine rites by Prince Vladimir I of Kyiv in 988 when there was established the Ecumenical Patriarch of Constantinople Metropolis of Kyiv and all Ruthenia. Later at the time of the Great Schism (ca 1054) the Ruthenian (Rusyn) Church took sides and remained Orthodox.