St. Augustine reminds us that our truest profession of faith is the way we love one another. One essential part of loving one another is working for unity in the Church. If we love Christ, we love his body, the Church.
Whoever therefore has not charity denies that Christ has come in the flesh. Here then do you now question all heretics. “Did Christ come in the flesh?” “He did come; this I believe, this I confess.” “Nay, this you deny.” “How do I deny? You hear that I say it!” “Nay, I convict you of denying it. You say with the voice, but deny with the heart; say in words, deny in deeds.”
How, do you say, do I deny in deeds? Because the end for which Christ came in the flesh, was, that He might die for us. He died for us, because therein He taught much charity. Greater charity than this has no man, that a man lay down his life for his friends. You have not charity, seeing you for your own honor divide the unity.
Therefore by this understand the spirit that is from God. Give the earthen vessels a tap, put them to the test, whether perhaps they be cracked and give a dull sound: see whether they ring full and clear, see whether charity be there. You take yourself away from the unity of the whole earth, you divide the Church by schisms, you rend the Body of Christ.
He came in the flesh, to gather in one, you make an outcry to scatter abroad. This then is the Spirit of God, which says that Jesus has come in the flesh, which says, not in tongue but in deeds, which says, not by making a noise but by loving. And that spirit is not of God, which denies that Jesus Christ has come in the flesh; denies, here also, not in tongue but in life; not in words but in deeds. It is manifest therefore how we may know the brethren.
–St. Augustine of Hippo, Homily 6 on First John
But Augustine also points out that another part of loving the Church is loving those who are not in the Church. The only way we bring them into the Church is by love, and the only way we truly love the Church is by loving those who are outside of the Church.
Love all men, even your enemies, not because they are your brethren, but that they may be your brethren; that you may be at all times on fire with brotherly love, whether toward him that has become your brother, or towards your enemy, so that, by being beloved, he may become your brother.
Wherever you love a brother, you love a friend. Now is he with you, now is he knit to you in unity, yea catholic unity. If you are living aright, you love a brother made out of an enemy. But you love some man who has not yet believed Christ, or, if he has believed, believes as do the devils: you rebuke his vanity. Love, and that with a brotherly love: he is not yet a brother, but you love in order that he may be a brother.
Well then, all our love is a brotherly love, towards Christians, towards all His members. The discipline of charity, my brethren, its strength, flowers, fruit, beauty, pleasantness, food, drink, meat, embracing, has in it no satiety. If it so delight us while in a strange land, in our own country how shall we rejoice!
–St. Augustine of Hippo, Sermon 10 on First John