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. For example, he says: Why do you judge your brother or why do you despise your brother? And again: You perform iniquity and common fraud, and this against your brothers.
From a Discourse on the Psalms by St Augustine. A Google search suggests it is about Psalm 32, but I cannot find this quote anywhere outside Augustine. I’m going to try to find the Latin text, and search that in case it is a translation thing, but he could also be slightly misquoting it.
So which prophet is he quoting, who spoke of people saying “you are not our brothers”?