With all the many Christian doctrines that are contained in the Bible it is important to make sure that we categorize them into a format that is easy to understand and also keeps us from dividing over things that need not divide us. Albert Mohler's article does such that in that it gives us a basic structure upon which to think critically about doctrine and then we can sort it into 1st, 2nd and 3rd level issues as necessary depending on things like how important it is to the historic Christian faith, as well as how clearly it is taught in the Scriptures.
In this article Mohler does an excellent job of placing the correct doctrines in the 1st and 2nd levels and then giving a good idea of why everything else should be relegated to the 3rd level. Things that find their way into the 3rd level should never divide any good faithful Christians. However, it will be seen that in the 2nd level there may be some need for separation of intimate fellowship and then at the 1st level there is need for complete separation.
The 1st level is by far the most important for it contains the fundamental doctrines of the historic Christian faith including:
Within this structure as Mohler points out and as I include when I teach this subject matter on the opposite ends of the spectrum are two groups that affect the theological triage in negative ways. The 1st is fundamentalism which takes 3rd level issues like translation choice and raises it up to the importance of a 1st level issue. On the opposite end of the spectrum is liberalism which lowers the importance of 1st level issues down to the same rank as 3rd level issues thus relegating the deity of Christ as no more important than a discussion of eschatology or translation choice.
So as you read the article think about the doctrines discussed and where they are placed and then think about how you would interact with somebody when discussing these subjects. We need not divide over something that is not on the level of a 1st level issue, and we should only divide over 2nd level issues in terms of the intimate fellowship that exists within a local body. For instance, divisions over modes of baptism, etc.
Below is the link to the article: