David strapped his sword over the tunic and tried to walk, but he was not accustomed to them. “I cannot walk in these,” David said to Saul. “I am not accustomed to them.” So David took them off.
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 David girded his, sword upon his armour,.... Which Saul also perhaps furnished him with: and he assayed to go; made an attempt, and had a mind to go thus accoutred; he at first showed an inclination to go in such an habit, but afterwards would not: for he had not proved it; as warriors were wont to do; so Achilles did (i); he never...
I cannot go with these - In ancient times it required considerable exercise and training to make a man expert in the use of such heavy armor; armor which in the present day scarcely a man is to be found who is able to carry; and so it must have been then, until that practice which arises from frequent use had made the proprietor perfect.