But Saul replied, “You cannot go out against this Philistine to fight him. You are just a boy, and he has been a warrior from his youth.”
David is at length presented to Saul for his champion (Sa1 17:31) and he bravely undertakes to fight the Philistine (Sa1 17:32): Let no man's heart fail because of him. It would have reflected too much upon the valour of his prince if he had said, Let not thy heart fail; therefore he speaks generally: Let no man's heart fail.
Commenting on 1 Samuel 17:31-39
And Saul said to David, thou art not able to go against this Philistine to fight with him,.... Had neither strength of body, nor skill in military affairs, to encounter with a man of his stature, and warlike genius and practice: for thou art but a youth; some say about fourteen or sixteen years of age, but very probably about twenty, and not more, and...
And Saul said to David, Thou art not able to go against this Philistine to fight with him: for thou [art but] a youth, and he a man of war from his youth. (k) Here Satan proves David's faith, by the infidelity of Saul.