Adam Clarke

Ye have not yet resisted unto blood - Many of those already mentioned were martyrs for the truth; they persevered unto death, and lost their lives in bearing testimony to the truth. Though you have had opposition and persecution, yet you have not been called, in bearing your testimony against sin and sinners, to seal the truth with your blood. Striving against sin - Προς την ἁμαρτιαν ανταγωνι ζομενοι· An allusion to boxing at the Grecian games. In the former passages the apostle principally refers to the foot races.