I suspect that "serenity" would not be on the top of too many people's lists if we asked them to name some of the characteristics of the Apostle Paul. Zeal? Of course. Ardor? Necessarily. Tirelessness? Without a doubt. But serenity? As in "to accept the things I cannot change"? Hmmm.
But that's just what today's first reading seems to indicate! Paul and Silas (in Philippi, a Roman colony, where Paul the Roman citizen should have been quite at home) have been beaten with rods (they literally got a "lickin' " by the lictores) and thrown, bleeding and sore, into a dungeon. And there, in the middle of the night, they sat, chained to a stake, "praying and singing hymns to God as the prisoners listened." Luke makes a distinction between the apostles (with their feet in chains) and "the prisoners." The apostles, praying and singing, were free men. Paul doesn't express a hint of frustration at being unable to "go out to all the world" to preach. He sits there and sings his heart out.
Interestingly, we find this same serenity, under the same sorts of circumstance, in the letters St. Paul wrote while "in chains for the Gospel." To the Philippians, who had witnessed his early imprisonment in their own city, he wrote from another prison in another city (probably Rome), "the circumstances of my imprisonment are helping to advance the spread of the Gospel. And so I rejoice!" What a contrast with the line in the Gospel, set before the Lord's suffering, death and resurrection: "grief has filled your hearts." Paul shows us that the Lord's promise had been fulfilled in him: "your grief will be turned into joy."
And see how well the often-neglected Entrance Antiphon sums it all up: Let us shout out our joy and happiness, and give glory to God the Lord of all, because he is our King, alleluia!"