John Chrysostom

They who drink bitter medicines, first submit to some unpleasantness, and afterwards feel the benefit. For such is virtue, such is vice. In the latter there is first the pleasure, then the despondency: in the former first the despondency, and then the pleasure. But there is no equality; for it is not the same, to be first grieved and afterwards pleased, and to be first pleased and afterwards grieved. How so? …

Homily on Hebrews 30 ↗

Theophylact of Ohrid

Again from a common thought he took occasion for persuasion and says: "all chastening," both divine and human, "seems not to be joy, but grief." He excellently says: "seems," for in reality it is not grief. For how could it properly be grief, being the mother of joy? But with reference to us, who are discontented with discipline, it is said: "seems." "But afterward it yields the peaceful fruit of righteousness to those who have been trained by it." What does "peaceful" mean? – that is, unshakeable, easy, pleasant. …

Commentary on Hebrews ↗

Cosmas Indicopleustes

For no chastening, saith scripture, seemeth for the present to be joyous but grievous: On those accordingly, who have been rationally tested, he has decreed to bestow afterwards in the future state his good things that are everlasting, and to fulfil what has been his primary purpose from the beginning—having taken, as God, a providential care of what concerns us, as became him, and as was for our advantage.

The Christian Topography, Book 5 ↗

Thomas Aquinas

681. – He continues: All discipline [chastisement] seems painful rather than pleasant; later it yields the peaceful fruit of righteousness [justice] to those who have been trained by it. This is the third reason, which is drawn from the benefit of the correction. But since chastisements are forms of medicine, the same judgement seems to be true of chastisement as of medicine. …

Commentary on Hebrews ↗