There's a moment in the Old Testament when the leader of God's people (King David) rallies all the people together to give what they can from their family treasure to the building of God's house so that more people could worship Him in splendor. And then He confesses something simple yet remarkable to God, "Who am I, and who are my people, that we could give anything to you? Everything we have has come from you, and we give you only what you have already given us!" (1 Chronicles 29:14). 

God has been generous to us, supremely through Jesus Christ, but also through the gift of energy and ability to work (Deuteronomy 8:18). Giving back to Him and His Kingdom work is an opportunity to respond to His generosity towards us. Out of love, we give back what He first gave to us.