John Calvin Commentary Genesis 19:22

"Haste thee, escape thither; for I cannot do anything till thou be come thither. Therefore the name of the city was called Zoar." — Genesis 19:22 (ASV)

I cannot do anything. Since the angel had not only been sent as an avenger to destroy Sodom but also had received a command for the preservation of Lot, he therefore declares that he will not perform the former act unless the latter is joined with it. This is because it is not within the servant's discretion to separate those things that God has joined together.

I am not, however, dissatisfied with the explanation of some who suppose the angel speaks in the person of God. For although the language may appear harsh, there is no absurdity in saying that God is unable to destroy the reprobate without saving His elect. Nor must we, therefore, consider His power to be limited when He places Himself under such a necessity, or think that any of His liberty and authority is diminished when He willingly and freely binds Himself.

And let us especially remember that His power is connected by a sacred bond with His grace and with faith in His promises. Hence, it may be truly and properly said that He can do nothing but what He wills and promises. This is a true and profitable doctrine. There will, however, be less reason for hesitation if we refer the passage to the angels, who had a positive commandment from which it was not lawful for them to deviate in the slightest.