Jamieson, Fausset and Brown on 1 Samuel 17:37

37. The Lord that delivered me--It would have been natural for a youth, and especially an Oriental youth, to make a parade of his gallantry. But David's piety sank all consideration of his own prowess and ascribed the success of those achievements to the divine aid, which he felt assured would not be withheld from him in a cause which so intimately concerned the safety and honor of His people.

Saul said unto David, Go, and the Lord be with thee--The pious language of the modest but valiant youth impressed the monarch's heart. He felt that it indicated the true military confidence for Israel, and, therefore, made up his mind, without any demur, to sanction a combat on which the fate of his kingdom depended, and with a champion supporting his interests apparently so unequal to the task.