Bp. of Seleucia-Ctesiphon, and cath. A native of Kashkar, Isḥaq was elected cath. in the first year of Yazdgard  I (r. 399–420), after a long period of persecution and hardship for the Christians in the Persian Empire. Relations between the Roman and Persian Empires improved at the end of the 4th cent. In this new political environment, Isḥaq was able to lay the foundation for the organization of the Christian communities, with the formal approval of the Persian authorities. In this process Isḥaq worked together with Bp. Marutha of Maypherqaṭ, an envoy from the Roman Empire. The Synod of 410, held in Seleucia-Ctesiphon, was the highlight of Isḥaq’s tenure. Its official account is preserved in the Synodicon Orientale. The text of the account is followed by a Creed, by 21 canons, and by 33 signatures of bishops. Some canons deal with the organization of the church, church discipline, and the relations among bishops. Along with the see of Seleucia-Ctesiphon (whose incumbent is given primacy ‘in all the regions of the East’, canon  12), the following five cities have metropolitan bishops (canon  21): Beth Lapaṭ (Beth Huzaye), Nisibis, Prat and Mayshan, Arbela, and Karka. Other canons contain instructions about liturgy (canon  9) and about the celebration of Christian feasts (canon 13). Some canons specifically aim at creating uniformity between East and West (i.e., Christianity within the Roman Empire) and parallels exist with canons of the Council of Nicea (325) and of other councils within the Roman Empire. In addition to the Synodicon Orientale, the creed and the canons of the synod are also preserved in the Syr. Orth. tradition (published by Lamy). For the creed, the latter has preserved a more original text, while the E.-Syr. text underwent later revision. The tone of the entire account is very optimistic with regard to both the peaceful coexistence of the Roman and Persian Empires and the freedom given to Christians to practice their religion and to organize themselves. Only a few years later, however, the situation of Christians in the Persian Empire again deteriorated. The synod of 419 (under Cath. Yahbalaha) clearly reflects the increased tension, while the Synod of 424 (under Cath. Dadishoʿ) was held in an atmosphere of crisis. As evidenced in the Synodicon Orientale, however, Isḥaq’s work retained its significance and continued to be seen as foundational for the organizational structure of the Ch. of E.