Charles Ellicott 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)

Certainly I will be with you. —Hebrew, since I will be with you. An answer addressed not to the thing said, but to the thing meant. Moses meant to urge that he was unfit for the mission. God’s reply is, “Not unfit, since I will be with you.” I will supply all your defects, make good all your shortcomings. “My strength is made perfect in weakness.”

This shall be a token to you. —It is in accordance with the Divine economy to give people “tokens,” which are future and appeal to faith only (Compare 1 Samuel 2:34; 2 Kings 19:27).