‘And you have forgotten the exhortation which speaks to you as to sons, “My son, regard not lightly the chastening (moral training, discipline) of the Lord, Nor faint when you are reproved by him, for whom the Lord loves he chastens, and scourges every son whom he receives.” '
He points out that they might have overlooked or forgotten the Biblical teaching on chastening and firm discipline as something by which God speaks to His children as to sons. They have clearly, in their concern to escape persecution, forgotten the exhortations of Scripture which had aided the past heroes and heroines of the faith to persevere. For example let them consider Proverbs 3:11-12, ‘My son, do not despise the chastening of the Lord, or faint when you are reproved by Him, for whom the Lord loves He chastens, and beats every son whom He receives'. This is almost word for word with LXX which merely excludes the ‘My'.
The warning here is against treating God's discipline and chastening as though it did not matter, or on the other hand, allowing it to affect them too much. Some shrug it off, others are devastated by it. But rather they must take it as an act of love from their Father, and learn from it the lesson that He wishes to teach them. Above all they must recognise that it is a sign of His love for them, demonstrating that He does care about what they are and what they become. It is a proof of His true Fatherhood.