2 Samuel 21:17
But Abishai the son of Zeruiah succoured him, and smote the Philistine, and killed him. Then the men of David sware unto him, saying, Thou shalt go no more out with us to battle, that thou quench not the light of Israel. But Abishai the son of Zeruiah succoured him, and smote the Philistine, and killed him. Then the men of David sware unto him, saying, Thou shalt go no more out with us to battle, that thou quench not the light of Israel.
The verse records one of the many victories in David's wars, when Abishai rescued David by smiting and killing the Philistine who threatened him. It was a rescue given when David was no longer safe to fight at the front. The following chapter's song ascribes the victory of such battles to God's goodness and power.
After the rescue the men of David made him swear a promise that he would go no more out with them to battle. They indeed forbade him to go, so that he would not quench the light of Israel. The light of Israel means its king and leader, its splendor and guiding light. He was the one on whom the people depended and whose life they sought to protect, so that his death would mean the loss of that light.
David is God's anointed light and a priest-king. He is a type of the heavenly Messiah, the anointed Son of God and King of the Jews. The chapter to which the verse belongs also teaches that God holds His people collectively responsible when many allow the sins of a few to continue. It shows that God hates oppression, even when it is practiced against people outside the household of faith.
The men's refusal to risk David was an existing ordinance. It would have kept David personally safe had he chosen war as the punishment for his census. Instead he chose pestilence, which exposed him and his household to the same danger as his subjects. Their action showed their care for David even when he believed he could still serve in battle.