God is just

Man to man and man to God is unjust, but God to man is just. He is fair in all his ways with man. We are justly rewarded for our actions and works. The bible reminds us of the justice of God. 
Galatians 6:7
Do not be deceived: God cannot be mocked. A man reaps what he sows.
Revelation 15:3
And sang the song of God's servant Moses and of the Lamb: "Great and marvelous are your deeds, LORD God Almighty. Just and true are your ways, King of the nations.