Jamieson, Fausset and Brown on Ecclesiastes 12:13

13. The grand inference of the whole book.

Fear God--The antidote to following creature idols, and "vanities," whether self-righteousness (Ec 7:16, 18), or wicked oppression and other evils (Ec 8:12, 13), or mad mirth (Ec 2:2; 7:2-5), or self-mortifying avarice (Ec 8:13, 17), or youth spent without God (Ec 11:9; 12:1).

this is the whole duty of man--literally, "this is the whole man," the full ideal of man, as originally contemplated, realized wholly by Jesus Christ alone; and, through Him, by saints now in part, hereafter perfectly (1Jo 3:22-24; Re 22:14).