because the Lord disciplines the one he loves, and he chastens everyone he accepts as his son.”
Endure hardship as discipline; God is treating you as his children. For what children are not disciplined by their father?
If you are not disciplined—and everyone undergoes discipline—then you are not legitimate, not true sons and daughters at all.
Moreover, we have all had human fathers who disciplined us and we respected them for it. How much more should we submit to the Father of spirits and live!