1 Samuel 18:17
And Saul said to David, Behold my elder daughter Merab, her will I give thee to wife: only be thou valiant for me, and fight the LORD’s battles. For Saul said, Let not mine hand be upon him, but let the hand of the Philistines be upon him.
Saul offered his elder daughter Merab to David as wife on condition that David be valiant for him and fight the Lord's battles against the Philistines. His hidden intent was not honor but murder by proxy, laying a snare so that David would be killed by the hand of the Philistines rather than by Saul's own hand. In this he sought to take David's life while still appearing justified before the nation. To carry out the plot he kept strict watch for any indiscretion or rashness in David that could be used as an excuse to bring him into disgrace, and he urged him to prosecute the war with still greater vigor, holding out alliance with the eldest daughter of the royal house as the reward for valor in order to expose him to death at their hands.
David modestly answered that he and his father's family in Israel were unworthy to become son-in-law to the king, and Saul proved his own insincerity by wedding the princess to another man. Fighting the Lord's battles means laboring earnestly, faithfully and unselfishly in God's great cause. It means enduring trial and reproach with courage and taking a firm stand on the side of God and the truth against the spirit of rebellion. Applied to Christians, it means representing Christ and enlisting in His service by using influence to bring others into that service, standing against the forces that seek to destroy God's image in the young, and setting an example sharply different from the practices of the evil age.