How well informed are you as a Christian? You know the Gospel, you know certain doctrines which you will defend, you read the Bible, but do you make it your business to understand the reason for everything that we do? When I was a young Christian we used to be guided by all kinds of rules. When you were converted they would sit you down and they would say, you don’t dance, you don’t smoke, you don’t do this, and you don’t do that. You witness for the Lord every day, you read your Bible, you pray, and here are your Christian duties. It was all set out. And then it seemed to fall by the wayside, and now you never hear the famous old dos and don’ts that young Christians used to be taught. We were helped by all those clear statements of the past. But the weakness of the old way was that people taught the rules and they did not teach why. It is contrary to Christian practice for this reason or for that reason - here it is in the Scripture. The reasons disappeared and soon the principles disappeared. Do you know the reasons why we do things as we do them? This is what the apostle is saying. Hold these things fast. Know why you believe all the things you believe. Know the practices that your church follows. Are they biblical? What is the reason for them? If you know the reason, it helps you to be loyal to the Lord, and to stand firm and to keep them. We should all be well informed and ready to enquire on everything so that we do it out of principle. Peter tells every Christian is to be ready to give a reason for the hope that is within him and to witness for the Lord constantly, all the time, every opportunity, praying for them.
The Roman Catholics understand this verse to teach that there is a quite separate verbal tradition which forms the basis of their own Catholic tradition over the centuries. Paul however is simply referring to the things he had taught the Thessalonians verbally which were no different to what he now wrote to them in his epistles. All that we need to know is delivered in Scripture.