Jamieson, Fausset and Brown on Galatians 6:3

3. Self-conceit, the chief hindrance to forbearance and sympathy towards our fellow men, must be laid aside.

something--possessed of some spiritual pre-eminence, so as to be exempt from the frailty of other men.

when he is nothing--The Greek is subjective: "Being, if he would come to himself, and look on the real fact, nothing" [Alford] (Ga 6:2, 6; Ro 12:3; 1Co 8:2).

deceiveth himself--literally, "he mentally deceives himself." Compare Jas 1:26, "deceiveth his own heart."