What I was particularly interested in was what the Fathers made of Jesus' command to treat the brother who sins against you and "refuses to listen even to the church... as you would a Gentile or a tax collector."
St. John Chrysostom points out that, when those who actually are Gentiles and tax collectors strike your cheek, you are to offer them your other cheek. St. Jerome concludes that
he is to be more abhorred, who under the name of a believer does the deeds of an unbeliever, than those that are openly gentiles.
St. Augustine points out that a Christian treats even Gentiles and tax collectors in the way a disciple of Jesus ought:
Though even thus we are not to neglect his salvation; for the heathens themselves, that is, the gentiles and pagans, we do not indeed regard in the number of our brethren, yet we ever seek their salvation.
We might then ask, how did Jesus treat Gentiles and tax collectors? How did He expect His disciples to treat them?