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1 Samuel 16:21ESV·author unknown

And David came to Saul and entered his service. And Saul loved him greatly, and he became his armor-bearer.

Matthew Henry Presbyterian @wholebiblehenry

We have here Saul falling and David rising. I. Here is Saul made a terror to himself (Sa1 16:14): The Spirit of the Lord departed from him. He having forsaken God and his duty, God, in a way of righteous judgment, withdrew from him those assistances of the good Spirit with which he was directed, animated, and encouraged in his government and wars. He lost all his good qualities.

Commenting on 1 Samuel 16:14-23

John Gill Reformed Baptist @doctorgill

And David came to Saul, and stood before him,.... As a servant, and ministered to him in the way, and for the purpose for which he was sent: and he loved him greatly; being a comely person, and a well behaved youth, and especially as he was serviceable to him with his music, in driving away melancholy from him: and he became his armourbearer; that...

Jamieson-Fausset-Brown Reformed @jfbcommentary

David came to Saul--Providence thus prepared David for his destiny, by placing him in a way to become acquainted with the manners of the court, the business of government, and the general state of the kingdom. became his armour-bearer--This choice, as being an expression of the king's partiality, shows how honorable the office was held to be.