The Messenger is coming—must come—because the LORD God has promised, and He is unchanging and always faithful no matter how unfaithful His people may be.
In many ways [this text] brings to mind Judgement Day and the separation of the sheep from the goats when Christ the King comes to take His treasured possession home to be with Him in the courts of everlasting life.
Even though the sins of the Israelites are great, like scarlet, they shall be as white as snow. In spite of great transgression, there is even greater forgiveness.