Theophylact of Bulgaria
If you are without chastening, of which all have become partakers, then you are illegitimate children and not sons. By the example of those about whom they thought that they had been abandoned by God, he shows them that they were the objects of God’s care.
If you were strangers to afflictions, it would appear that you were illegitimate children and not sons. For what father cares for the upbringing and chastity of illegitimate sons?
Since you are subjected to oppression, your life thereby appears chaste and temperate. So also all the righteous mentioned above are plainly righteous, and they were called sons of God.
It is clear that this does not contradict the fact that God deals with you as with sons and cares for you, lest, being undisciplined, you fall away, and lest for this He deprive you of the inheritance.