Albert Barnes Commentary Exodus 3:12

"And he said, Certainly I will be with thee; and this shall be the token unto thee, that I have sent thee: when thou hast brought forth the people out of Egypt, ye shall serve God upon this mountain." — Exodus 3:12 (ASV)

A token unto thee - Or the sign. The word means a declaration or promise of God, which rests absolutely on His word, and demands faith. The promise that God would have the people serve Him in that place was an assurance, if fully believed, that all intervening obstacles would be removed by His power.