The imperial court was displeased with the religious principles of Ambrose, however his aid was soon solicited by the Emperor. When
Magnus Maximus usurped the supreme power in
Gaul, and was meditating a descent upon Italy, Valentinian sent Ambrose to dissuade him from the undertaking, and the embassy was successful.
On a second attempt of the same kind Ambrose was again employed; and although he was unsuccessful, it cannot be doubted that, if his advice had been followed, the schemes of the usurper would have proved abortive; but the enemy was permitted to enter
Italy; and
Milan was taken. Justina and her son fled; but Ambrose remained at his post, and did good service to many of the sufferers by causing the plate of the church to be melted for their relief.
Ambrose was equally zealous in combating the attempt made by the upholders of the old state religion to resist the enactments of Christian emperors. The pagan party was led by
Quintus Aurelius Symmachus, consul in
391, who presented to
Valentinian II a forcible but unsuccessful petition praying for the restoration of the
Altar of Victory to its ancient station in the hall of the
Roman Senate, the proper support of seven
Vestal Virgins, and the regular observance of the other pagan ceremonies.
To this petition Ambrose replied in a letter to Valentinian, arguing that the devoted worshipers of
idols had often been forsaken by their
deities; that the native valour of the Roman soldiers had gained their victories, and not the pretended influence of pagan
priests; that these idolatrous worshipers requested for themselves what they refused to Christians; that voluntary was more honourable than constrained
virginity; that as the Christian ministers declined to receive temporal emoluments, they should also be denied to pagan priests; that it was absurd to suppose that
God would inflict a famine upon the empire for neglecting to support a religious system contrary to His will as revealed in the
Holy Scriptures; that the whole process of nature encouraged innovations, and that all nations had permitted them even in religion; that heathen sacrifices were offensive to Christians; and that it was the duty of a Christian prince to suppress pagan ceremonies. In the epistles of Symmachus and of Ambrose both the petition and the reply are preserved.
To support the logic of his argument, Ambrose halted the celebration of the
Eucharist, essentially holding the Christian community hostage, until Theodosius agreed to abort the investigation without requiring reparations to be made by the bishop.
Theodosius I, the emperor of the East, espoused the cause of Justina, and regained the kingdom. Theodosius was threatened with excommunication by Ambrose for the
massacre of 7,000 persons at
Thessalonica in
390, after the murder of the Roman governor there by rioters. Ambrose told Theodosius to imitate
David in his repentance as he had imitated him in guilt - Ambrose readmitted the emperor to the Eucharist only after several months of penance . This incident shows the strong position of a bishop in the Western part of the empire, even when facing a strong emperor - the controversy of
John Chrysostom with a much weaker emperor a few years later in Constantinople led to a crushing defeat of the bishop.
Ambrose's influence upon Theodosius is credited with eliciting the enactment of the "Theodosian decrees" of
391 (see entry
Theodosius I).
In
392, after the death of
Valentinian II and the acclamation of
Eugenius, Ambrose supplicated the emperor for the pardon of those who had supported Eugenius after Theodosius was eventually victorious. Soon after acquiring the undisputed possession of the
Roman empire, Theodosius died at Milan in
395, and two years later (
April 4,
397) Ambrose also died. He was succeeded as bishop of Milan by
Simplician. Ambrose's body may still be viewed in the church of S. Ambrogio in Milan, where it has been continuously venerated — along with the bodies identified in his time as being those of
Sts. Gervase and Protase — and is one of the oldest extant bodies of historical personages known outside
Egypt.