Episcopate
As bishop of the see of Tarsus, Diodore continued to speak out for the Nicene understanding of the relationship between the human and the divine in the person of Jesus Christ. He actively opposed both the Arianism and the Apollinarianism of his day (Arius taught that Jesus Christ was not fully divine, Apollinaris of Laodicea that he was not fully human).
Diodore played key roles in both the local Council of Antioch in 379 and the ecumenical First Council of Constantinople in 381. When their mentor Meletius died in 381, Diodore recommended his friend Flavian as his successor, thus prolonging the division in the Antiochene church.
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