The key to explaining right division is for people to see the differences in your Bible.
For example, the mystery truth of the one body is that there is neither Jew nor Greek in Christ Jesus (Galatians 3:28; 1 Corinthians 12:13).
This mystery fellowship makes no difference between Jew and Gentile. Yet, this mystery is not meaningful until the difference between Jew and Gentile in time past is respected and made clear.
In attempt to show someone the difference it is easy to say, “that book was written to Israel”, or “that was for the Jews not Gentiles”, but these statements fall flat on the person who does not understand the difference.
They already assume there is no difference today, because they are ignorant of the difference in time past. This is why they take every instruction in the Bible as theirs. It may not be a failure to rightly divide, it is that they did not know there was a difference to divide.
You need to help them see by explaining that there was a big difference that existed before. Jew and Gentile were different: God gave special privileges and promises to the nation of Israel and not to Gentiles.
Here how Paul does it, “Remember, that ye being in time past…” (Eph 2:11).
Only when they respect what God did differently in time past, will they see clearly why the fellowship unity we have now is different. It is different than what God was doing before. This is the beginning of right division.
Justin “respect the differences” Johnson