Here is a good, short article by John Piper reflecting on Ephesians 6:7-8. Here’s an excerpt from his fourth point, “Encouragement that nothing good is done in vain”:
Perhaps the most amazing sentence of all is this: “Whatever good thing each one does, this he will receive back from the Lord.” This is amazing. Everything. Every little thing you do that is good is seen and valued by the Lord. And he will pay you back for it. Not in the sense that you have earned anything by putting him in your debt. He owns you and everything in the universe. He owes us nothing. But he freely, graciously chooses to reward good things done in faith. Nothing we do. Nothing. Not one thing is done in vain. “Whatever good thing each one does, this he will receive back from the Lord.” Astounding!