John MacArthur has a 3 part series at the Pulpit Magazine blog on what doctrines Christians can consider to be fundamental to our faith.
In the first post (which I commented on here) he makes the statements that all fundamental doctrines are found in scripture, and are clear in scripture.
In his second post he gives more guidelines:
1. Everything essential to saving faith Is fundamental
In his comments on this point he says:
a doctrine must be regarded as fundamental if eternal life depends on it. Scripture is full of statements that identify the terms of salvation and the marks of genuine faith.
2. Every doctrine we are forbidden to deny is fundamental
On this he writes:
Certain teachings of Scripture carry threats of damnation to those who deny them. Other ideas are expressly stated to be affirmed only by unbelievers. Such doctrines, obviously, involve fundamental articles of genuine Christianity
In the third post he gives one more guideline:
The fundamental doctrines are all summed up in the person and work of Christ
So — he gives 5 guidelines to follow to determine if a doctrine is fundamental or not:
1. All fundamental doctrines are found in scripture
2. All fundamental doctrines are clear in scripture
3. Everything essential to saving faith Is fundamental
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