What we know about the celebrated Alexandrian theologian, the last head of the school for catechists in that city, who was blind from early youth. This treatment summarizes his works as then known, including many disputable treatise, like the Against Arius and Sabellius found under the name of Gregory of Nyssa. Leipoldt summarizes Didymus' character, culture, and literary style, and assesses his moral theology and his wider theological views: he was a solid trinitarian, but influenced by the merciful views of Origen that all mankind would be saved. He was condemned for this, posthumously, under Justinian, which is why most of his voluminous writings have perished.