17. not as many—(2Co 11:18; Php 2:21). Rather, "the many," namely, the false teachers of whom he treats (tenth through twelfth chapters, especially 2Co 11:13; 1Th 2:3).
which corrupt—Greek, "adulterating, as hucksters do wine for gain" (2Co 4:2; Isa 1:22; 2Pe 2:3, "Make merchandise of you").
as of sincerity … as of God—as one speaking from (out of) sincerity, as from (that is, by the command of, and so in dependence on) God.
in Christ's—as united to Him in living membership, and doing His work (compare 2Co 12:19). The whole Gospel must be delivered such as it is, without concession to men's corruptions, and without selfish aims, if it is to be blessed with success (Ac 20:27).