Sproul’s third practical rule for interpreting scripture is:

Historical Narratives are to be Interpreted by the Didactic

The best example of this is the New Testament.  The Gospels (Matthew, Mark, Luke, and John) record the historical narrative on the work and life of Jesus.  While they do show Jesus teaching, the epistles are didactic in nature (didactic refers to something that teaches).

Sproul points out that the Epistles teach us and help us better understand the Gospels.  However, this is not an absolute rule.

As you read the Gospels and come across something that is not entirely clear, search the epistles and you will find clarity.  Remember, there is no contradiction in the word of God.