[1]: 35–37 He was a candidate to succeed Atticus upon the latter's death in 425, but the populace favoured the elderly Sisinnius I of Constantinople (426–427) for his lack of affiliation with local rivalries and his charity to the poor.

A traditional title that had been in use for over a century and utilised by preceding bishops of Constantinople like Gregory of Nazianzus, Theotokos was by this point deeply associated with the liturgical life of the capital.

[1]: 52–53  Although he was initially reticent to get involved in the emerging dispute, when a monastic deacon named Basil submitted a formal accusation of heresy to the Emperor against the Archbishop, Proclus withdrew from communion with Nestorius.

In 430, on the occasion of a Feast Day of the Theotokos that was probably instituted under Atticus for the Nativity Cycle, Proclus was invited by Nestorius to deliver a homily from the pulpit before a mixed crowd.

Although he did not attend the Council of Ephesus at which Nestorius was subsequently deposed, Proclus was the recipient of a letter from the conciliar president and leader of the anti-Nestorian party, Cyril of Alexandria, in which his contribution was warmly acknowledged.

[1]: 71–77 By the time Archbishop Maximianus died on Great and Holy Thursday of 434, the Imperial government had firmly come down on the side of the anti-Nestorians and were keen to avoid further electoral disturbances.

[2] His archepiscopacy was dominated by the aftermath of the Council of Ephesus, and his consecration was rejected by the portion of Eastern bishops who had seceded from communion with John of Antioch after his reunion with Cyril.

The whole affair showed conspicuously the moderation and tact of Proclus and resulted in a period of peace for the Churches of the East after the Imperial Commission secured a forced union through exiling Nestorius and his closest supporters.

[1]: 90–91 In continuity with the conciliation of Atticus in the latter years of his episcopacy, Proclus oversaw the transfer of the relics of Saint John Chrysostom from Comana Pontica back to Constantinople in 438,[1]: 114–115 [4] where he interred them with great honour in the Church of the Twelve apostles.

He also continued the trend of his predecessors in expanding both the construction of churches, shrines and monasteries suitable to house growing numbers of relics and monks, but also the jurisdictional reach of the See of Constantinople.

[1]: 9–10 Proclus' homilies present clear affirmations of the unity and duality of the natures of Christ in terminology Constas describes as "anticipating the language of Chalcedon".

His mentor Archbishop Atticus made important contributions to the development of early Byzantine veneration of the Virgin in the two decades prior to the Council of Ephesus, chiefly through the establishment of the first officially sanctioned Marian feast.

[1]: 38  Proclus' famous Homily 1 delivered in Constantinople in 430 "defined the rhetoric and rationale for the cult of the Virgin Mary throughout the Byzantine period", "attained de facto canonical status" after it was attached to the proceedings of the Ephesine Council, and was the basis of centuries of theological re-imagining and reflection.

The text interweaves resplendent praises of Mary with celebration of female virginity, directly confronting Nestorius' two-pronged criticisms of local Marian piety and the Empress Pulcheria.

[5] One of the primary texts he cites to support this is Ezekiel 44:1–2, interpreting the closed gate of the sanctuary typologically, in continuity with 4th-century figures like the Cappadocians who laid the foundation for the rhetorical praise of the sacred womb.

He argues on the basis of theological content that the hymn was already extant by 430 and that Proclus was quoting it, which indicates pre-existing Marian piety between Jerusalem and Constantinople upon which the bishop drew for his rhetorical purposes.