See what a crown he has framed for him by bearing witness to such great hospitality in him, and brought in the entire Church into this man's house! For by the word ξένον, used here, he means a host, not a guest. But when you hear that he was Paul's host, do not admire him for his munificence only, but also for his strictness of life. For except he were worthy of Paul's excellency, he would never have lodged there, since he, who took pains to go beyond many of Christ's commands, would never have trespassed against that law, which bids us be very particular about who receive us, and about lodging with “worthy” persons. “Erastus, the chamberlain of the city, salutes you, and Quartus a brother.” There is a purpose in his adding “the chamberlain of the city,” for as he wrote to the Philippians, “They of Cæsar's household salute you”, that he might show that the Gospel had taken a hold upon great folk, so here too he mentions the title with a view to the same object, and to show that, to the man who gives heed, neither riches are a hindrance, nor the cares of government, nor anything else of the kind.
Ver. 24. “The grace of our Lord Jesus Christ be with you all. Amen.”
See what we ought to begin and to end with everywhere! For in this he laid the foundation of the Epistle, and in this he puts on the roof, at once praying for the mother of all good things for them, and calling the whole of his loving-kindness to their mind. For this is the best proof of a generous teacher, to benefit his learners not by word only, but likewise by prayer, for which cause also one said, “But let us give ourselves continually to prayers, and to the ministry of the word.”
Source: Homilies on Romans (New Advent)