For if a man think himself to be something, when he is nothing, he deceiveth himself.
Note 5 at Ga 6:3: Paul was saying that if we think we are too important to stoop down and help others with their burdens, then we are deceived about our own importance. None of us are anything of ourselves. None of us have any good excuse for not helping restore our fellow believers. This was one of the sins of the Pharisees (see note 4 at Lu 18:9).
The Amplified Bible translates this verse as, "For if any person thinks himself to be somebody [too important to condescend to shoulder another's load] when he is nobody [of superiority except in his own estimation], he deceives and deludes and cheats himself."