Action taken in light of the promises of God yields results - big results.  Here, Jonathan's seemingly rash and crazy solo attack on a Philistine outpost turns into a massive victory for the nation of Israel, as a "panic sent by God" (vs. 15) engulfs the Philistine army.

To me, this passage shows sharp contrast between Saul and his son Jonathan.  The nation of Israel stands on the brink of destruction, threatened by a massive force of enemy Philistines, and without modern weapons with which to fight.  To make matters worse, the Israelite army has dropped considerably in numbers, from 3,000 men a short time ago (chapter 13) to just 600.  In the face of these overwhelming odds, Saul and Jonathan both make choices - one to trust in God's faithfulness and deliverance enough to take action, and one to sit in inaction.