The Lord was with David and had gone ahead of Him in the battle, but, David knew that he would have to go out and in obedience and adaptability physically claim the victory. We don't see that the Lord had spoken to him directly, but, that His anointing had emboldened the lad along with the prior experience fighting lion and bear. Others had tried to outfit him as they saw fit to protect him, in this particular case their armor was a hinderence.
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