At the moment of purification, Isaiah hears and responds to Yahweh’s call for a messenger. This scene is eerily similar to the Savior’s response to the premortal call for a redeemer: “And the Lord said: Whom shall I send? And one answered like unto the Son of Man: Here am I, send me. And another answered and said: Here am I, send me. And the Lord said: I will send the first” (Abr. 3:27).
Yahweh’s call is not a lack of knowledge about who his special messengers will be. Rather it is an affirmation of the principle of agency. Yahweh so fully respects agency that such calls are not commands but invitations. The prophet’s acceptance is his free-will offering of service.