Jamieson, Fausset and Brown on 1 Corinthians 1:6

6. According as the testimony of (of, and concerning) Christ (who is both the object and author of this testimony [Bengel]; 1Co 2:1; 1Ti 2:6; 2Ti 1:8) was confirmed among [Alford] you; that is, by God, through my preaching and through the miracles accompanying it (1Co 12:3; Mr 16:20; 2Co 1:21, 22; Ga 3:2, 5; Ep 4:7, 8; He 2:4). God confirmed (compare Php 1:7; He 2:3), or gave effect to the Gospel among (or better as English Version, "in") the Corinthians by their accepting it and setting their seal to its truth, through the inward power of His Spirit, and the outward gifts and miracles accompanying it [Calvin].