1 Samuel 29-31

After David and his men lived under the rule of Achish for a long time the Philistines prepared to battle Israel again. Achish sought to include David's group among the Philistine fighters but the other kings refused; fearful that David would turn on them. Achish must had been impacted by David's faith because when he proclaimed an oath he leaned on the God of Israel. Maybe he heard David sing of his love for the God from whom he had been cast out. Maybe he had heard the intensity of David's prayers. Maybe he was just being kind to David. Whatever the reason the God whom David worshiped had receive the notice of one of the Philisitine kings.
Stuart