Greek Orthodox metropolitan of Philadelphia and theological polemicist and writer on Christian asceticism, who emerged as a central figure in the political and theological turmoil of his age.

A married deacon of the Eastern Church, in Bithynia, northwest Asia Minor, Theoleptus actively opposed the attempted church union of East and West promoted by the political strategy of the Byzantine emperor Michael VIII Palaeologus. With the majority of the Orthodox clergy, he repudiated the temporary decree of union promulgated by John XI Beccus, patriarch of Constantinople, and Pope Gregory X at the general council of Lyon in 1274, and consequently was excommunicated by Beccus. Leaving his wife, Theoleptus retired into solitude c. 1275, probably on Mt. Athos, northwest Greece, the centre of Eastern Orthodox monasticism. There he practiced a form of contemplative prayer known as Hesychasm.

At the accession of the anti-unionist emperor Andronicus II (1282–1328) and patriarch Gregory I, reunion with Rome was formally rejected. Named archbishop of Philadelphia in 1285, Theoleptus composed a sharp critique of the Orthodox clergy who supported patriarch Beccus' unionist movement. His attack on patriarch Gregory II Cyprius' moderate doctrinal teaching on the Holy Spirit's relationship to both Father and Son caused Gregory's dismissal. Renowned as a spiritual director throughout Byzantium, Theoleptus was chosen by Empress Irene, widow of Emperor John Palaeologus, to be her adviser.