"Fight the good fight of the faith." 1 Timothy 6:12a
A true soldier fights for something greater than himself. As Christians, we have been given the privilege to fight (and possibly die) for the greatest cause possible: to give glory to Jesus Christ.
King David understood this. Prior to becoming king, David faced Goliath not so that he could attain great wealth, fame, power or gloating material, but he insisted to battle Goliath so that "all the earth may know that there is a God in Israel" (1 Samuel 17:46).
Without ceasing, we are constantly battling Satan and our own flesh. These battles will continue to our last breath whilst we have our physical body as it is stained by sin, but let us persevere and remember that we do not live ourselves.
We were created and redeemed to glorify God. Let every thought, action and breath reflect the fact that we live and fight for Christ.