His three years' stay in Constantinople was wearisome and otherwise disagreeable; the leisure it forced upon him he devoted in part to literary composition. The Aegyptus sive de providentia is an allegory in which the good Osiris and the evil Typhon, who represent Aurelian and the Goth Gainas (ministers under Arcadius), strive for mastery; and the question of the divine permission of evil is handled.
After the successful Aurelian had granted the petition of the embassy, Synesius returned to Cyrene in 400, and spent the next ten years partly in that city, when unavoidable business called him there, but chiefly on an estate in the interior of the province, where in his own words "books and the chase" made up his life. His marriage took place at Alexandria in 403; in the previous year he had visited Athens.
In 409 or 410 Synesius, whose Christianity had until then been by no means very pronounced, was popularly chosen to be bishop of Ptolemais, and, after long hesitation on personal and doctrinal grounds, he ultimately accepted the office thus thrust upon him, being consecrated by Theophilus at Alexandria. One personal difficulty at least was obviated by his being allowed to retain his wife, to whom he was much attached; but as regarded orthodoxy he expressly stipulated for personal freedom to dissent on the questions of the soul's creation, a literal resurrection, and the final destruction of the world, while at the same time he agreed to make some concession to popular views in his public teaching.
His tenure of the bishopric was troubled not only by domestic bereavements (his three sons died) but also by barbaric invasions of the country (in repelling which he proved himself a capable military organizer) and by conflicts with the praeses Andronicus, whom he excommunicated for interfering with the Church's right of asylum. The date of his death is unknown; it is usually given as c. 414, because he appears to have been unaware of the violent death of Hypatia.
His many-sided activity, as shown especially in his letters, and his loosely mediating position between Neoplatonism and Christianity, make him a subject of fascinating interest. His scientific interests are attested by his letter to Hypatia in which occurs the earliest known reference to a hydrometer, and by a work on alchemy in the form of a commentary on pseudo-Democritus.