Children (no matter how old!) should always honour their parents (Eph. 6:1-3). This can be especially difficult when a parent’s life has aspects that are ungodly. Jonathan, David’s friend and King Saul’s son, was someone who had to do exactly that: honour a parent who was often living in an ungodly way.
In many parts of his life (e.g. when he was trying to kill David), Saul wasn’t a godly person. Jonathan didn’t support his father in these ungodly things (e.g. he supported David [1Sam. 19:1-5], often to his father’s annoyance [1Sam. 20:30-33]); in fact, he tried to help his father see the the problems in his life and choose instead the right way (sometimes it worked [1Sam. 19:6], and sometimes it didn’t [1Sam. 20:30-33]). However, in all other aspects of his life, Jonathan still stood by his father right up to (and even into!) his death; in the words of David:
Saul and Jonathan, beloved and lovely! In life and in death they were not divided; they were swifter than eagles; they were stronger than lions. [2Sam. 1:23]
Perhaps the following verse might be useful, too:
Do not rebuke an older man but encourage him as you would a father… [1Tim. 5:1]
This doesn’t mean that a child should never try to “correct” a parent who is straying from a godly life; rather, it’s about how to go about doing it. The NASB translates the verse like this:1
Do not sharply rebuke an older man, but rather appeal to him as a father… [1Tim. 5:1 (NASB)]
Similarly, 2 Tim. 4:2 says ‘reprove, rebuke, and exhort, with complete patience and teaching’.
Honouring, whilst gently correcting, parents who are straying from God’s way of life is a difficult, but beautiful, course to take.
1. Compare NRSV: ‘Do not speak harshly to an older man’ (NLT also); NET: ‘Do not address an older man harshly’.