John Chrysostom

"Unto the Church of God." Not "of this or of that man," but of God. "Which is at Corinth." Seest thou how at each word he puts down their swelling pride; training their thoughts in every way for heaven? He calls it, too, the Church "of God;" shewing that it ought to be united. For if it be "of God," it is united, and it is one, not in Corinth only, but also in all the world: for the Church's name (exxlhsia: properly an assembly) is not a name of separation, but of unity and concord. …

Homily on 1 Corinthians 1 ↗

Theophylact of Ohrid

"To the church of God which is at Corinth." The Church is not of this or that person, but of God. How then do you have men as your leaders? If, again, you are the Church, then you must be in perfect unity. "To those sanctified in Christ Jesus." You were sanctified not in some human being, but in Christ, that is, through baptism, and not through wisdom or wealth, of which you boast. "Called to be saints." And that very thing, he says, that you believed, is not from you, but you obeyed and believed because you were called by God. …

Commentary on 1 Corinthians ↗

Thomas Aquinas

Then he mentions the persons he is greeting, saying: to the church of God that is at Corinth. First, he mentions the chief persons, whom he describes in three ways: first, from their region when he says, to the church of God that is at Corinth, i.e., Christ's faithful assembled at Corinth: "I will thank thee in the great congregation" (Ps 35:18). …

Commentary on 1 Corinthians ↗