In a previous post I quoted from Aquinas concerning the difference between faith and individualism. Here I wish to comment briefly on Aquinas's understanding of the nature of unity, and apply it to the Church. Aquinas was well-studied in the works of previous philosophers on the subject of unity. He drew deeply, for instance, from Aristotle's arguments in the Metaphysics (Bk 4.2 and Bk 10). There Aristotle shows the relation of unity and being, and the different kinds and degrees of unity. In his Summa Theologica, Aquinas explains the relation of unity and being:
"One" does not add any reality to "being"; but is only a negation of division; for "one" means undivided "being." This is the very reason why "one" is the same as "being." Now every being is either simple or compound. But what is simple is undivided, both actually and potentially. Whereas what is compound, has not being whilst its parts are divided, but after they make up and compose it. Hence it is manifest that the being of anything consists in undivision; and hence it is that everything guards its unity as it guards its being.
If we understand the relation of being and unity, we recognize that insofar as a thing loses unity, it loses being, and insofar as a thing gains unity, a thing gains being. This is why, for example, a house divided against itself cannot stand (St. Matthew 12:25; St. Luke 11:17). Jesus says this in the context of defending His casting out of demons, as we hear in today's gospel reading. One way of building up the Church, and thus giving it greater being or presence in the world, is to strengthen or increase its unity.
In the revelation of Jesus, we learn that God is love (1 John 4:8). And in this way we understand that love and unity are interconnected, as I discussed recently here and here. Love, within the one God in three Persons of the Blessed Trinity, is the greatest form of unity. Aquinas thus treated the sins of discord, contention, and schism as sins against charity. In relation to schism, Aquinas discusses the role of the pope in authoritatively determining the articles of faith, i.e. doctrine. Aquinas writes:
Now this belongs to the authority of the Sovereign Pontiff, "to whom the more important and more difficult questions that arise in the Church are referred," as stated in the Decretals [Dist. xvii, Can. 5. Hence our Lord said to Peter whom he made Sovereign Pontiff (Luke 22:32): "I have prayed for thee," Peter, "that thy faith fail not, and thou, being once converted, confirm thy brethren." The reason of this is that there should be but one faith of the whole Church, according to 1 Corinthians 1:10: "That you all speak the same thing, and that there be no schisms among you": and this could not be secured unless any question of faith that may arise be decided by him who presides over the whole Church, so that the whole Church may hold firmly to his decision. Consequently it belongs to the sole authority of the Sovereign Pontiff to publish a new edition of the symbol, as do all other matters which concern the whole Church, such as to convoke a general council and so forth.
Here Aquinas shows from Scripture the significance of Christ's words to St. Peter, "I have prayed for thee, that thy faith fail not." This verse signifies something not only about Peter, but about his office. Next Aquinas shows why Jesus prayed this prayer for Peter, so that there should be but one faith of the whole Church. In other words, since we know whose faith will not fail, we know who to look to in determining what and where is the true faith. In this way, we can avoid and overcome schisms.
St. Thomas Aquinas, please pray for us, for the reconstitution of the full and visible unity of all Christ's followers. In the name of the Father, and of the Son, and of the Holy Spirit. Amen.