And David said to Saul, Let no man’s heart fail because of him; thy servant will go and fight with this Philistine.
And Saul armed David with his armour, and he put an helmet of brass upon his head; also he armed him with a coat of mail.
And David girded his sword upon his armour, and he assayed to go; for he had not proved it. And David said unto Saul, I cannot go with these; for I have not proved them. And David put them off him.
And he took his staff in his hand, and chose him five smooth stones out of the brook, and put them in a shepherd’s bag which he had, even in a scrip; and his sling was in his hand: and he drew near to the Philistine.

COMMENTARY

And David said to Saul, Let no man’s heart fail; thy servant will go and fight with this Philistine
When Goliath taunted the Israelite armies there was no one in the ranks willing to face him. Israel was in need of a heroic warrior but no one was answering the call. Now once upon a time that hero might have been King Saul himself, but Saul’s great warrior days were long passed. So someone new was needed to take the mantle, and it was into this vacancy that David entered.

And Saul armed David with his armour, and he put an helmet of brass upon his head
And David said unto Saul, I cannot go with these; And David put them off him
And he took his staff in his hand, and chose him five smooth stones out of the brook
Saul was grateful to receive David into this heroic role, and tried to give him his own tools of the trade. But they were ill-fitting for David and he couldn’t make do with them. Saul was dressing up David to fight the giant in close-combat. Maybe that approach would have worked for Saul, but Saul wasn’t the one going to fight the giant. David was, and David worked better at range. David defended his flocks with sling and stone, not sword and shield.
This was David’s calling now and he needed to go about it is own way, with the strengths he had developed himself, with the skills God had bestowed on him.
There is a great temptation for mentors to hover over, to micromanage, and to make sure things are done the “right way.” But so long as the protégé never finds their own “right” way, they can never realize their full potential. Sooner or later everyone else must leave the room and leave the would-be hero alone with God. That is when one really learns what they are made of.