Today's first reading starts with Paul and his missionary companion Silas jailed in Philippi. Luke specifies that they had not only been thrown into prison, but put in the innermost cell with their feet chained to a stake. There, Luke says, they sat "praying and singing hymns to God as the prisoners listened."
I find it interesting that Luke didn't say "as the other prisoners listened." It is as if Paul and Silas were the only free men in Philippi, chains or no chains. Today's Gospel tells us of the source of their freedom: "the ruler of this world has been condemned."