Pulpit Magazine has three interesting posts from John MacArthur on what he feels are essential doctrines that should not be compromised. It was helpful to me because rather than just stating a list of what he feels are essential, he provides a method for determining if a doctrine should be essential. Here is a quick overview:

1. All Fundamental Articles of Faith Must Be Drawn from the Scriptures
2. The Fundamentals Are Clear in Scripture
3. Everything Essential to Saving Faith Is Essential
4. Every Doctrine We Are Forbidden to Deny Is Fundamental
5. The Fundamental Doctrines Are All Summed up in the Person and Work of Christ

Read the posts here:

• Part I
• Part II
• Part III