Moses has asked for a sign. God gives him a sign stranger than any wonder.
"But He said, Therefore My Memra shall be for thy help; and this shall be the sign to thee that I have sent thee: when thou hast brought the people forth from Mizraim, ye shall worship before the Lord, because ye shall have received the Law upon this mountain."
The Targum Pseudo-Jonathan on Exodus (3:12) preserves one of the strangest signs in the Torah. A sign is supposed to happen before the task, to confirm the mission. This sign happens after. Moses is being told: the proof I sent you is that you will return here, to this same mountain, with the whole nation at your back, and receive the Torah. You will know I was speaking the truth today when you are standing on this ground in fifteen months with three million witnesses.
This is a theology of faith. The Holy One is asking Moses to act on a promise whose verification lies on the far side of the very action being requested. You cannot know the sign is true until you have already trusted it. This is what the sages will later call emunah — not intellectual belief, but the kind of walking that commits before it sees.
And the sign itself is extraordinary. God does not say "the sign is that you will be safe." He does not say "the sign is that Pharaoh will yield." The sign is: you will be back here to receive the Law. The purpose of the Exodus is being announced at the bush. The deliverance is prelude. Sinai is the point.
Beloved, many tasks will not be verified until they are completed. Walk anyway. The Memra walks with you.