To me was given the grace to preach the inscrutable riches of Christ. The Church celebrates today St. Ambrose of Milan. Among the many things for which St. Ambrose is renowned, perhaps the one that tops the list is the role he played in the conversion and eventual baptism of St. Augustine. As the bishop of Milan, St. Ambrose distinguished himself as a compelling preacher and defender of orthodoxy. Along with Sts. Jerome, Augustine and Gregory I, Ambrose is one of the four great doctors of the Latin Church. 
In the First Reading, St. Paul is telling the Ephesians that his own unmerited call to preach to, and boldly evangelize the Gentiles is a highpoint in a much larger mysterious plan of God, that is, the coming to knowledge of God by the Gentiles. Whereas St. Ambrose cannot claim to be St. Paul, he too was able to play a part in the continued unveiling of God’s plan to the nations. He accomplished this both as a preacher and a bishop. His dedication as a preacher led to conversion to faith of many souls, including bringing Theodosius to public penance. As a bishop, St. Ambrose understood that it was his responsibility not only to preach the Word but also to ensure that he shepherded to eternal life the flock that the Lord had put under his charge. Like the good shepherd about whom we have read in the Gospel passage, the wellbeing of the flock was Ambrose’s priority. It was for this reason that he fiercely fought the heretical teaching of Arianism.