Pope Gregory I
Apocrisiariate (579–585)

In 579, Pelagius II chose Gregory as his apocrisiarius (ambassador to the imperial court in Constantinople), a post Gregory would hold until 586. Gregory was part of the Roman delegation (both lay and clerical) that arrived in Constantinople in 578 to ask the emperor for military aid against the Lombards. With the Byzantine military focused on the East, these entreaties proved unsuccessful; in 584, Pelagius II wrote to Gregory as apocrisiarius, detailing the hardships that Rome was experiencing under the Lombards and asking him to ask Emperor Maurice to send a relief force. Maurice, however, had long ago determined to limit his efforts against the Lombards to intrigue and diplomacy, pitting the Franks against them. It soon became obvious to Gregory that the Byzantine emperors were unlikely to send such a force, given their more immediate difficulties with the Persians in the East and the Avars and Slavs to the North.

According to Ekonomou, "if Gregory's principal task was to plead Rome's cause before the emperor, there seems to have been little left for him to do once imperial policy toward Italy became evident. Papal representatives who pressed their claims with excessive vigor could quickly become a nuisance and find themselves excluded from the imperial presence altogether". Gregory had already drawn an imperial rebuke for his lengthy canonical writings on the subject of the legitimacy of John III Scholasticus, who had occupied the Patriarchate of Constantinople for twelve years prior to the return of Eutychius (who had been driven out by Justinian). Gregory turned to cultivating connections with the Byzantine elite of the city, where he became extremely popular with the city's upper class, "especially aristocratic women". Ekonomou surmises that "while Gregory may have become spiritual father to a large and important segment of Constantinople's aristocracy, this relationship did not significantly advance the interests of Rome before the emperor". Although the writings of John the Deacon claim that Gregory "labored diligently for the relief of Italy", there is no evidence that his tenure accomplished much towards any of the objectives of Pelagius II.

Gregory's theological disputes with Patriarch Eutychius would leave a "bitter taste for the theological speculation of the East" with Gregory that continued to influence him well into his own papacy. According to Western sources, Gregory's very public debate with Eutychius culminated in an exchange before Tiberius II where Gregory cited a biblical passage in support of the view that Christ was corporeal and palpable after his Resurrection; allegedly as a result of this exchange, Tiberius II ordered Eutychius's writings burned. Ekonomou views this argument, though exaggerated in Western sources, as Gregory's "one achievement of an otherwise fruitless apokrisiariat ". In reality, Gregory was forced to rely on Scripture because he could not read the untranslated Greek authoritative works.

Gregory left Constantinople for Rome in 585, returning to his monastery on the Caelian Hill. Gregory was elected by acclamation to succeed Pelagius II in 590, when the latter died of the plague spreading through the city. Gregory was approved by an Imperial iussio from Constantinople the following September (as was the norm during the Byzantine Papacy).
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