Albert Barnes on Galatians 6:3

For if a man think himself to be something, etc. See Gal 5:26. This is designed, evidently, to be another reason why we should be kind and tender to those who have erred. It is, that even those who are most confident may fall. They who feel secure, and think it impossible that they should sin, are not safe. They may be wholly deceived, and may be nothing, when they have the highest estimate of themselves. They may themselves fall into sin, and have need of all the sympathy and kindness of their brethren.

When he is nothing. When he has no strength, and no moral worth. When he is not such as he apprehends, but is lifted up with vain self-conceit.

He deceiveth himself. He understands not his own character. "The worst part of the fraud falls on his own head."--Doddridge. He does not accomplish what he expected to do; and instead of acquiring reputation from others, as he expected, he renders himself contemptible in their sight.