Hebrews 12:8

‘But if you are without chastening, of which all have been made partakers, then are you bastards, and not sons.'

Indeed God's disciplining and chastening is a sign of high favour. It is the true born son who is disciplined and chastened because the father is concerned to train him properly with a view to his future responsibilities. He is an heir and therefore proper concern must be shown for his upbringing. He bears the family name. What he becomes is important. It is the illegitimate children, who will have no rights to inherit, who have no name to uphold, who can be left with no proper training, so that they can behave as they like. So it is if they find themselves without chastening that they need to be concerned, not when they are chastened, for not to be chastened will simply demonstrate that they are not true believers, true born sons at all.

(This is not to be taken as God's views on illegitimate children. The writer is using an illustration from how things were at the time, not passing a judgment on whether it was right or not).