George Eldon Ladd, A Theology of the New Testament, Eerdmans, 1993, p. 589.
Paul [addressed] the Corinthians as “those sanctified in Christ Jesus, called to be saints” (1 Cor. 1:2). The congregation in Corinth was anything but a “holy” people in terms of life and conduct; false teaching, schisms, and immorality marred the church. Still, it was a congregation of saints, of the sanctified, because in spite of the sinful conduct of many of its members and the worldly character of the church itself, it was still the church of God in Corinth.
George Eldon Ladd, A Theology of the New Testament, Eerdmans, 1993, p. 589.