God considers that “the imagination of man’s heart is evil from his youth” (Gen 8:21) when he announces he will never “smite any more every thing living”; but this is the same reasoning he used to destroy man (see Gen 6:5). What are we to make of this?

This is discussed above in connection to Gen 6:5, but let us return to the issue. Often such puzzles are solvable only if one assumes that the text is written with a surprising amount of subtlety. Fortunately, the text of the Bible bears up under such scrutiny; it really does have a surprising amount of subtlety. In this case, we are to compare what God says about “the imagination of man’s heart”—it is “evil from his youth”—to what he says about the antediluvians—“every imagination of the thoughts of [their] heart was only evil continually” (6:5); the three words I italicized here represent a significant difference. Man henceforth will, or at least should, be restrained by such practices as sacrifice, law, and awareness of God’s covenant and frightening sovereignty. He might still be “evil from his youth,” and so deserving of damnation; but at least his every thought will not be only evil continually. That is progress.