John Calvin Commentary Hebrews 10:35

"Cast not away therefore your boldness, which hath great recompense of reward." — Hebrews 10:35 (ASV)

Cast not away, therefore, etc. He shows what especially makes us strong to persevere: namely, retaining confidence. For when that is lost, we lose the recompense set before us. Therefore, it appears that confidence is the foundation of a godly and holy life.

By mentioning reward, he diminishes nothing from the gratuitous promise of Salvation. For the faithful know that their labor is not in vain in the Lord, while they still rest on God’s mercy alone. But it has often been stated elsewhere how reward is not incompatible with the gratuitous imputation of righteousness.