Gaius mine host, and of the whole church, saluteth you. Erastus the chamberlain of the city saluteth you, and Quartus a brother. Gaius mine host - Gaius in Greek is the same as Caius in Latin, which was a very common name among the Romans. St. Luke (Acts 19:29) mentions one Gaius of Macedonia, who was exposed to much violence at Ephesus in the tumult excited by Demetrius the silversmith against St. Paul and his companions; and it is very possible that this was the same person. He is here called not only the host ξενος, the entertainer of St. Paul, or Tertius, (if he wrote this and the following verse), but also of the whole Church: that is, he received and lodged the apostles who came from different places, as well as the messengers of the Churches. All made his house their home; and he must have been a person of considerable property to be able to bear this expense; and of much piety and love to the cause of Christ, else he had not employed that property in this way.
Erastus the chamberlain of the city - Treasurer of the city of Corinth, from which St. Paul wrote this epistle. This is supposed to be the same person as is mentioned Acts 19:22. He was one of St. Paul's companions, and, as appears from 2 Timothy 4:20, was left about this time by the apostle at Corinth. He is called the chamberlain οικονομος, which signifies the same as treasurer; he to whom the receipt and expenditure of the public money were intrusted. He received the tolls, customs, etc., belonging to the city, and out of them paid the public expenses. Such persons were in very high credit; and if Erastus was at this time treasurer, it would appear that Christianity was then in considerable repute in Corinth. But if the Erastus of the Acts was the same with the Erastus mentioned here, it is not likely that he now held the office, for this could not at all comport with his travelling with St. Paul. Hence several, both ancients and moderns, who believe the identity of the persons, suppose that Erastus was not now treasurer, but that having formerly been so he still retained the title. Chrysostom thought that he still retained the employment.
Quartus a brother - Whether the brother of Erastus or of Tertius we know not; probably nothing more is meant than that he was a Christian - one of the heavenly family, a brother in the Lord.