the servant of King Saul of Israel: To Achish, David appeared to be a valuable defector (someone who had left his former king). The other Philistine rulers were suspicious. They feared that David was a spy who planned to attack them from inside their defenses.
found no fault in him: This was the first of three times Achish defended David (see also 1 Samuel 29:6, 9). However, David had been deceiving Achish the entire time (see 1 Samuel 27:1–12).