Main point: Serve God, let the outcome be up to Him.
In this chapter, Joab takes Israel to war against Ammon and Syria. As they are about to fight, verse 14 is recorded. In it he tells them to fight valiantly for God and let Him do whatever is good in His sight. They understood two things: 1) They were going to give their all for God and 2) They were not promised prosperity or victory.
I need to remember this. When I serve God, I should not expect a certain out come. My service and love for God should not be motivated by a desired result. I should live completely for God and understand that what He decides to do with my life is good. I do not know what God wants to do with my life, wether I will live to serve Him for a hundred years or maybe die tomorrow. Either way, I must fight valiantly for Him. All I want is what He wants, and that may mean destroying me personally.
This shows the gospel in showing God’s control. In salvation, man does not have ultimate control. Man can do all he wants, even after salvation, but it will come down to God for salvation.