So David said to Achish, "But what have I done? And to this day what have you found in your servant as long as I have been with you, that I may not go and fight against the enemies of my lord the king?" 1 Samuel 29:8
Fleeing from King Saul, David took refuge with King Achish of the Philistines. In the Bible, Israel pictures an individual Christian, while the Philistines picture besetting sin (sin that won't go away). While serving Achish, David secretly raided Israel's enemies, while lying about his activities. Not knowing David's true motives, King Achish took David and his army with him when the Philistines went to battle agaisnt God's people. Fortunately, the other lords of the Philistines didn't trust David, and they successfully demanded that David be sent away.
Now, Brethren, EVERY commentary I can find says that David hadn't wanted to fight against King Saul, and that he lied to Achish when he pretended that he did. But there's another explanation:
David meant what he said. After years of trying to get along with God's annointed servant, David finally decided to fight him. It really is hard to believe that King Achish would take David to fight God's people without talking to him first. And Achish really did believe that David could be trusted to fight against King Saul.
In 2 Timothy, we are told "3 You therefore must endure hardship as a good soldier of Jesus Christ.
4 No one engaged in warfare entangles himself with the affairs of this life, that he may please him who enlisted him as a soldier."
David was a soldier, and it was God Who had made him into one. David fought often, and he fought successfully. After years of fighting God's enemies, he finally decided to use the skills God had given him to fight God's allies.
Unknown to David, while he was on his way to fight God's people, God's enemies raided his city and took the people captive. What had gone wrong? David had gone to fight the wrong people. If he had been fighting God's battles, he wouldn't have had time to fight for the Devil.
When Christians forsake soul-wining, witnessing, teaching God's Word, etc., they have time to serve as soldiers against other Christians. As a Christian, you might find other Christians attacking you. Hopefully, you'll be too busy serving God to fight back.