St. Augustine

The prophet refers to some men saying: When they say to you: You are not our brothers, you are to tell them: You are our brothers. Consider whom he intended by these words. Were they the pagans? Hardly; for nowhere either in Scripture or in our traditional manner of speaking do we find them called our brothers. Nor could it refer to the Jews, who do not believe in Christ. Read Saint Paul and you will see that when he speaks of "brothers," without any qualification, he refers always to Christians. If they say, "Why do you seek us? What do you want of us?" we should reply: You are our brothers. They may say, "Leave us alone. We have nothing to do with you." But we have everything to do with you, for we are one in our belief in Christ; and so we should be in one body, under one head.