God, displeased with what had been done, spoke to Samuel, saying that he repented that he had made Saul king. The priest reports what he had heard to the king. And ere long, being instructed by God, he anointed David with the royal oil, while he was as yet only a little boy living under the care of his father, and acting as a shepherd, while he was accustomed often to play upon the harp. For this reason, he was taken afterwards by Saul, and reckoned among the servants of the king.
And the Philistines and Hebrews being at this time hotly engaged in war, as the armies were stationed opposite to each other, a certain man of the Philistines named Goliath, a man of marvelous size and strength, passing along the ranks of his countrymen, cast insults, in the fiercest terms, upon the enemy, and challenged any one to engage in single combat with him. Then the king promised a great reward and his daughter in marriage to any one who should bring home the spoils of that boaster; but no one out of so great a multitude ventured to make the attempt.
In these circumstances, though still a youth, David offered himself for the contest, and rejecting the arms by which his yet tender age was weighed down, simply with a staff and five stones which he had taken, advanced to the battle. And by the first blow, having discharged one of the stones from a sling, he overthrew the Philistine; then he cut off the head of his conquered foe, carried off his spoils, and afterwards laid up his sword in the temple. In the meanwhile, all the Philistines, turning to flight, yielded the victory to the Hebrews.
But the great favor shown to David as they were returning from the battle excited the envy of the king. Fearing, however, that if he put to death one so beloved by all, that might give rise to hatred against himself and prove disastrous, he resolved, under an appearance of doing him honor, to expose him to danger. First then he made him a captain, that he might be charged with the affairs of war; and next, although he had promised him his daughter, he broke his word, and gave her to another.
Ere long, a younger daughter of the king, Melchol by name, fell violently in love with David. Accordingly, Saul sets before David as the condition of obtaining her in marriage the following proposal: that if he should bring in a hundred foreskins of the enemy, the royal maiden would be given him in marriage; for he hoped that the youth, venturing on so great dangers, would probably perish. But the result proved very different from what he imagined, for David, according to the proposal made to him, speedily brought in a hundred foreskins of the Philistines; and thus he obtained the daughter of the king in marriage.
Source: Sacred History (New Advent)