The Didactic Must Interpret the Historical
Another rule stemming from this principle that the Bible is progressive revelation is that the Gospels and Acts (The historical) are interpreted, for the most part, by the Epistles or didactic (teaching epistles). The emphasis of the Gospels and Acts is found generally in the record of events, while the Epistles are generally more concerned with interpretation of the significance of these events in terms of doctrine, exhortation and application. If we are confused, then, about something in the Gospels or Acts, we ought to check on what the Epistles have to say about the issue. The teaching epistles or the didactic will help us to interpret the narrative portions.
This rule is not absolute, especially when the Gospels and Acts have teaching recorded. For instance, in many of the passages in Matthew, Mark, Luke, John and Acts, not only is there a record of the acts of Jesus and the apostles, but teaching recorded as well. Does that mean that Jesus' teaching and the apostles' teaching in the Gospels and Acts is given less authority than that of the Epistles (the letters of the apostles)? That is not the intent of this principle. Whenever the teaching of Jesus or the apostles is recorded, it is authoritative.
This rule is important; however, to warn against drawing too many references from records of what people do in the narrative passages.