Jamieson, Fausset and Brown on Ecclesiastes 9:18

18. one sinner, &c.--(Jos 7:1, 11, 12). Though wisdom excels folly (Ec 9:16; 7:19), yet a "little folly (equivalent to sin) can destroy much good," both in himself (Ec 10:1; Jas 2:10) and in others. "Wisdom" must, from the antithesis to "sinner," mean religion. Thus typically, the "little city" may be applied to the Church (Lu 12:32; He 12:22); the great king to Satan (Joh 12:31); the despised poor wise man, Jesus Christ (Is 53:2, 3; Mr 6:3; 2Co 8:9; Ep 1:7, 8; Col 2:3).