Tensions in Christian unity started to become evident in the 4th century. Two basic problems were involved: the nature of the primacy of the bishop of Rome and the theological implications of adding a clause to the Nicene Creed, known as the filioque clause. These doctrinal issues were first openly discussed in Photius's patriarchate. The Eastern churches viewed Rome's understanding of the nature of episcopal power as being in direct opposition to the Church's essentially conciliar structure and thus saw the two ecclesiologies as mutually antithetical.
Another issue developed into a major irritant to Eastern Christendom, the gradual introduction into the Nicene Creed in the West of the Filioque clause – meaning "and the Son" – as in "the Holy Spirit ... proceeds from the Father and the Son", where the original Creed, sanctioned by the councils and still used today by the Eastern Orthodox, simply states "the Holy Spirit, ... proceeds from the Father." The Eastern Church argued that the phrase had been added unilaterally and therefore illegitimately, since the East had never been consulted. In addition to this ecclesiological issue, the Eastern Church also considered the Filioque clause unacceptable on dogmatic grounds.