1 Corinthians 1:2

the church of God--He calls it so notwithstanding its many blots. Fanatics and sectaries vainly think to anticipate the final sifting of the wheat and tares (Mat 13:27-30). It is a dangerous temptation to think there is no church where there is not apparent perfect purity. He who thinks so, must at last separate from all others and think himself the only holy man in the world, or establish a peculiar sect with a few hypocrites. It was enough for Paul in recognizing the Corinthians as a church, that he saw among them evangelical doctrine, baptism, and the Lord's Supper" [CALVIN]. It was the Church of God, not of this or of that favorite leader [CHRYSOSTOM]. at Corinth--a church at dissolute Corinth--what a paradox of grace! sanctified--consecrated, or set apart as holy to God in (by union with) Christ Jesus. In the Greek there are no words "to them that are"; translate simply, "men sanctified." called to be saints--rather, "called saints"; saints by calling: applied by Paul to all professing members of the Church. As "sanctified in Christ" implies the fountain sources of holiness, the believer's original sanctification in Christ (Co1 6:11; Heb 10:10, Heb 10:14; Pe1 1:2) in the purposes of God's grace, so "called saints" refers to their actual call (Rom 8:30), and the end of that call that they should be holy (Pe1 1:15). with all that in every place call upon . . .

— Jamieson-Fausset-Brown Commentary (public domain)