The oath a witness takes in court is to “tell the truth, the whole truth, and nothing but the truth, so help me God.”
This oath comes from the commandment not to bear false witness.
You might think it is redundant, but not so. Someone can tell the truth and still bear false witness, by not telling the whole truth.
Someone can tell the truth, and by adding something to it make it a lie.
The same oath is a good pattern for teaching the Bible as well.
If your teaching emphasizes one verse and neglects a hundred others that contradict, then you may be telling a truth, but not the whole truth. (The devil does this.)
If you are teaching philosophy, legends, tradition, or special translation you have departed from speaking nothing but the truth of scripture. It does not become true, just because it is quoted alongside a true Bible verse. (Scholars do this often.)
Many times when people disagree with you about what the Bible says it is because they have a different witness of truth. If two witnesses fundamentally disagree, someone is not telling the truth, the whole truth, and nothing but the truth.
Your job is to seek it out.
For His glory,
Justin “raise your right hand” Johnson