From a discourse on the psalms by Saint Augustine, bishop

Whether they like it or not, those who are outside the church are our brothers

We entreat you, brothers, as earnestly as we are able, to have charity, not only for one another, but also for those who are outside the Church. Of these some are still pagans, who have not yet made an act of faith in Christ. Others are separated, insofar as they are joined with us in professing faith in Christ, our head, but are yet divided from the unity of his body. My friends, we must grieve over these as over our brothers; and they will only cease to be so when they no longer say our Father.

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.

Those then who tell us: You are not our brothers, are saying that we are pagans. That is why they want to baptise us again, claiming that we do not have what they can give. Hence their error of denying that we are their brothers. Why then did the prophet tell us: Say to them: You are our brothers? It is because we acknowledge in them that which we do not repeat. By not recognising our baptism, they deny that we are their brothers; on the other hand, when we do not repeat their baptism but acknowledge it to be our own, we are saying to them: You are our brothers.

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.

And so, dear brothers, we entreat you on their behalf, in the name of the very source of our love, by whose milk we are nourished, and whose bread is our strength, in the name of Christ our Lord and his gentle love. For it is time now for us to show them great love and abundant compassion by praying to God for them. May he one day give them a clear mind to repent and to realise that they have nothing now but the sickness of their hatred, and the stronger they think they are, the weaker they become. We entreat you then to pray for them, for they are weak, given to the wisdom of the flesh, to fleshly and carnal things, but yet they are our brothers. They celebrate the same sacraments as we, not indeed with us, but still the same. They respond with the same Amen, not with us, but still the same. And so pour out your hearts for them in prayer to God.