Charles Ellicott Commentary 1 Samuel 29:9

"And Achish answered and said to David, I know that thou art good in my sight, as an angel of God: notwithstanding the princes of the Philistines have said, He shall not go up with us to the battle." — 1 Samuel 29:9 (ASV)

As an angel of God. —Again a simile, which Achish most likely borrowed from Hebrew thought, and made use of in his graceful courtesy as likely to be acceptable to David.