The following passages do not refer to FLEETING thoughts but thoughts not resisted which embrace evil IMAGINATIONS! However, such IMAGINATIONS are "evil" and "sin" regardless if they are acted out in the manifest life or not. Saved people are no more imune from "evil thoughts" as lost people. Indeed, insomuch that Jesus could actually tell an apostles "get thee behind me Satan."
Mt. 15:19 For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies:
Mt. 5:28 But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.
Ge 6:5 And GOD saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually.
Ge 8:21 And the LORD smelled a sweet savour; and the LORD said in his heart, I will not again curse the ground any more for man’s sake; for the imagination of man’s heart is evil from his youth; neither will I again smite any more every thing living, as I have done.
Pr 24:9 The thought of foolishness is sin: and the scorner is an abomination to men.
Jer 17:9 The heart is deceitful above all things, and desperately wicked: who can know it?
Notice Genesis 6:5 describes the whole mindset of man as "EVIL CONTINUALLY" and that is the cause of "the wickedness of man was great in the earth." This describes the MORAL STATE OF THEIR MINDS AND HEARTS.
Notice that Gen. 8:21 describes a characteristic MIND SET "from his youth" until his manhood. This is a characterization of man AFTER the flood and man IN GENERAL! Depraved nature.