Summary: We must fight against evil and against complacency and fear. We must flee from evil and protect our kids.
In the animal kingdom, the fight-or-flight response involves stress. Animals are inclined to fight or flee impending dangers such as predators and environmental stressors. For humans, the response is based upon an infinite number of factors, from fears to friends, from habits to hang-ups. And as is true in the animal kingdom, sometimes fleeing isn't so much an indication of weakness but of wit.
Sometimes we have to choose whether to stand up and fight a powerful enemy or wisely flee a wicked temptation. In the case of fatherhood, fight or flight is what you teach your kids, not what you do to them. There is perhaps no better (or worse) picture of this than is played out in the familiar story of David and Goliath.
BODY OF SERMON
(Note to pastor: As you transition into the body of your message, summarize the story of Israel's monarchy.)
Following Israel's occupancy of the promised land and Joshua's death, a new generation was born who did not know God or the mighty things He had done (Judg. 2:10).
A horrible cycle of disobedience began where Israel would rebel, be oppressed, cry out to God, and then be rescued by God via the judges, men and women who were called to speak God's truth and rescue God's people. The Book of Judges ends with the startling words, "In those days there was no king in Israel; everyone did whatever he wanted" (Judg. 21:25).
It was moral anarchy and led to a time of struggle. The Book of Ruth begins with famine and ends with the birth of several soon-to-be kings.
First Samuel ushered in the monarchy. The prophets and the priests protected the prescribed worship of God in days when God's word and prophetic visions were scarce (1 Sam. 3:1). And in chapter 8, against the warnings of their faithful prophet Samuel, the people demanded a king. To Samuel's dismay, the Lord commanded him to give in to their demands (1 Sam. 8:1-22).
There was no one more impressive or qualified than Saul, who became Israel's first anointed king--and first rejected king because of disobedience to the Lord. David was named Saul's successor; however, before David could become king, Saul would have to cease and desist, which was unlikely. Not as unlikely as the future king being placed in the current king's court as a soothing harp player, but that's what happened nonetheless (1 Sam 16:14-23).
Being challenged and engaged in all-out war, Israel's face-off with the Philistines brought on a new kind of battle. It would be survival of the fittest, a fight to the death, winner's tribe takes all--the Philistine fighter Goliath versus any man of Israel who dared.
(Note to pastor: Highlight Saul's fear and the fear of Israel. Compare and contrast 1 Samuel 9:2 (quickview)  and the description of Saul and 1 Samuel 17:4-7 (quickview)  with its description of Goliath.)
Israel's best was afraid of the Philistine's worst. Physical appearance had already been proven important to the Israelites (see the description of Saul in 1 Sam 9 and the admonition of the Lord in 1 Sam. 16) so naturally, the giant would have been an object of great fear.