Jamieson, Fausset and Brown on Hebrews 12:4

4. not yet resisted unto blood--image from pugilism, as he previously had the image of a race, both being taken from the great national Greek games. Ye have suffered the loss of goods, and been a gazing-stock by reproaches and afflictions; ye have not shed your blood (see on He 13:7). "The athlete who hath seen his own blood, and who, though cast down by his opponent, does not let his spirits be cast down, who as often as he hath fallen hath risen the more determined, goes down to the encounter with great hope" [Seneca].

against sin--Sin is personified as an adversary; sin, whether within you, leading you to spare your blood, or in our adversaries, leading them to shed it, if they cannot through your faithfulness even unto blood, induce you to apostatize.