The story of David/Hazrat Dawood (A.S) and the Goliath, the giant is related in sacred texts of people of the book, while as the testaments narrate the story in high tones, Holy Quran assumes it to relate the motto, the maxim involved in story:
‘’By Allah’s will, they routed them, and David slew Goliath; and Allah gave him power and wisdom and taught him whatever (else) He willed. And did not Allah check one set of people by another, the earth would indeed be full of mischief: but Allah is full of bounty to all the worlds’’ (Al Baqarah: 251)
Hazrat Dawood (A.S) was young, nevertheless endowed with wisdom, bravery, and an uncanny manner to depend on his own faculties.
As Israelites with Saul/Hazrat Talut (A.S) in command faced Philistines with the giant Goliath in their ranks, Saul offered to provide to provide his kingly armour to David.
He refused, preferring his own sling and staff, with which he would tend and defend his flock of sheep. Hazrat Dawood (A.S) floored the giant and then dealt the final blow with the sword of the giant.
The motto, the maxim is obvious, in defending the national objectives, which in case of David v/s Goliath was to defend his people—Israelites against the might of Goliath, one may depend on his own faculties, don his own armour, and preserve faith.
The bare fact that sword of Goliath was used by David to deliver the final blow makes the point that given the Divine Will, enemy’s armour might come handy to execute the rout.
The Holy Verse while relating the maxim also holds the universal truth—while Allah (SwT) is full of bounty to all the worlds, He does hold in check one set of people by another.
This is another motto that the story of David v/s Goliath holds. In this case Philistines were held in check by Israelites to undo the mischief.
In due course of events Hazrat Dawood (A.S) assumed the kingship of Israelites after Hazrat Talut (A.S). He was followed by Solomon/Hazrat Suleiman (A.S). All the three were Kings as well as Prophets of Israelites.