John Calvin Commentary Hebrews 12:11

"All chastening seemeth for the present to be not joyous but grievous; yet afterward it yieldeth peaceable fruit unto them that have been exercised thereby, [even the fruit] of righteousness." — Hebrews 12:11 (ASV)

Now no chastening, etc. This he adds, lest we should measure God’s chastisements by our present feelings. For he shows that we are like children who dread the rod and shun it as much as they can, because, owing to their age, they cannot yet judge how useful it may be to them.

The purpose, then, of this admonition is that chastisements cannot be assessed correctly if judged according to what the flesh feels under them, and that therefore we must fix our eyes on the end: we will thus receive the peaceable fruit of righteousness.

And by the fruit of righteousness he means the fear of the Lord and a godly and holy life, of which the cross is the teacher. He calls it peaceable because in adversities we are alarmed and disquieted, being tempted by impatience, which is always noisy and restless. But being chastened, we acknowledge with a resigned mind how profitable that which previously seemed bitter and grievous became to us.