Mar Eshai Shimun XXI (Syriac: ܡܪܝ ܐܝܫܝ ܫܡܥܘܢ ܟܓ.; 26 February 1908 – 6 November 1975), sometimes known as Mar Eshai Shimun XXIII, Mar Shimun XXIII Ishaya, Mar Shimun Ishai, or Simon Jesse, served as the 119th Catholicos-Patriarch of the Church of the East from 1920, when he was a youth, until his murder on 6 November 1975.
The Assyrian Church of the East (ACOE), sometimes called the Church of the East and officially known as the Holy Apostolic Catholic Assyrian Church of the East, is an Eastern Syriac Christian denomination that follows the traditional Christology and ecclesiology of the historical Church of the East. It belongs to the eastern branch of Syriac Christianity, and employs the Divine Liturgy of Saints Addai and Mari belonging to the East Syriac Rite. Its main liturgical language is Classical Syriac, a dialect of Eastern Aramaic. Officially known as the Church of the East until 1976, it was then renamed the Assyrian Church of the East, with its patriarchate remaining hereditary until the death of Shimun XXI Eshai in 1975.
The Assyrian Church of the East is officially headquartered in the city of Erbil, in northern Iraq; its original area encompassed Iraq, southeastern Turkey, northeastern Syria and northwestern Iran, corresponding roughly to ancient Assyria. The current Catholicos-Patriarch of the Assyrian Church of the East, Mar Awa III, was consecrated in September 2021.