2 Peter 3:16 (NKJV)
In 2 Peter 3:16 there is an interesting combination of words, "untaught" and "unstable". The word "untaught" comes from a Greek word meaning ignorant or uninformed, describing a person who does not know something. The word "unstable" means "unfixed", giving us the picture of someone who is not settled in one place, someone who lacks a firm foundation. Peter did not say untaught and stable, or taught and unstable and I think that is an important point to notice, Peter connects being unstable with being untaught. You may be a nice person but if you are untaught you will probably be unstable.
Peter is saying that some untaught and unstable people had taken things that Paul had written and "twisted" them, resulting in their own destruction. It would seem that if those same people had been taught correctly that they would have been more stable and they would have been less likely to have misunderstood or misinterpreted what Paul wrote. Usually we don't have problems with the things we know about, it is the things we don't know about that we have problems with. Many Christians are very wonderful people who love God but they are uniformed about what the Bible has to say.
It is sad to say but many Christians have not been taught well and they tend to be unstable. Too often people are like what Paul described in Ephesians 4:14 where he said "that we should no longer be children tossed to and fro and carried about with every wind of doctrine"; that is a picture of an unstable person. God needs stable, mature people to carry our His plans and purposes in the earth today. God's Word gives us a firm foundation to live a successful life. We need to regularly spend time with God's Word so that we are not untaught or unstable and we don't destroy ourselves or others.