Surely thou shalt know what thy servant can do - This was another equivocal answer; and could only be understood by his succeeding conduct. It might imply what he could do in favor of the Philistines against Israel; or in favor of Israel against the Philistines. Achish understood it in the former sense; and therefore he said to David, I will make thee keeper of my head for ever; i.e., Thou shalt be captain of my life-guards.
Thou shalt know - David dissembled (compare also 1 Samuel 29:8), hoping, no doubt, that something would happen to prevent his fighting against his king and country.
Keeper of mine head - Captain of his bodyguard.
“And it came to pass in those days, that the Philistines gathered their armies together for warfare, to fight with Israel. And Achish said unto David, Know thou assuredly, that thou shalt go out with me to battle, thou and thy men.” David had no intention of lifting his hand against his people; but he was not certain as to what course he would pursue, until circumstances should indicate his duty. He answered the king evasively, and said, “Surely thou shalt know what thy servant can do.” Achish understood these words as a promise of assistance in the approaching war, and pledged his word to bestow upon David great honor, and give him a high position at the Philistine court. PP 674.1
But although David's faith had staggered somewhat at the promises of God, he still remembered that Samuel had anointed him king of Israel. He recalled the victories that God had given him over his enemies in the past. He reviewed the great mercy of God in preserving him from the hand of Saul, and determined not to betray a sacred trust. Even though the king of Israel had sought his life, he would not join his forces with the enemies of his people. PP 674.2
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