God trains my hands for battle

"David continued: “The same LORD who delivered me from the claws of the lion and the bear will deliver me from the hand of this Philistine.” Saul answered David, “Go! the LORD will be with you.”  (1 Samuel 17: 37)

If God did it once, God will repeat what He has done before.  David knew that his source of inspiration and power is God.  This awareness gives him considerable calm and courage.

"All this multitude, too, shall learn that it is not by sword or spear that the LORD saves. For the battle belongs to the LORD, who shall deliver you into our hands."  (1 Samuel 17: 47)

God is sovereign, almighty and the First Cause of everything.  He does not need spear or sword to affect any outcome.  "Let there be light."  And there was light.  "Stretch out your hand.'  He stretched it out and his hand was restored."  (Mark 3: 5)  If God wills David to overcome Goliath, He inspires the means and gives David the power to accomplish God's purpose.

Years later, David was to include in his memoir (the Psalms) the following thought:

"Blessed be the LORD, my rock,
who trains my hands for battle,
my fingers for war;

My safeguard and my fortress,
my stronghold, my deliverer,
My shield, in whom I take refuge,
who subdues peoples under me."  (Psalm 144: 1 - 2)

How does God train our hands for battle, our fingers for war.

The answer is plain in Hebrews 12: 7:  "Endure your trials as 'discipline.'"
Every disappointment, delay, pain, suffering is a blessing.  Our disappointment is God's appointment.  Our pain is our tutor.