Paul addresses Timothy:
Share in suffering as a good soldier of Christ Jesus. No soldier gets entangled in civilian pursuits, since his aim is to please the one who enlisted him. An athlete is not crowned unless he competes according to the rules. It is the hard-working farmer who ought to have the first share of the crops. Think over what I say, for the Lord will give you understanding in everything.
Remember Jesus Christ, risen from the dead, the offspring of David, as preached in my gospel, for which I am suffering, bound with chains as a criminal. But the word of God is not bound!
(2 Timothy 2:3-9)
A good soldier joins the fight for the faith, committing to be faithful to the cause of Christ, his brothers in the church, the church as his family, and the elders to whom he's accountable. He has the cause in view, understands the mission -- if not totally, at least to the extent of his role in it -- throws off distractions and entangling temptations, eager to please to whom and that to which he is pledged.
A good soldier follows the rules, not merely out of duty but out of his guts, out of an understanding of the importance of the rules. He doesn't just obey the Law, he delights in it, having lost his taste for the way of the world. He rejects passivity, puts his nose to the grindstone, gets his hands dirty, develops blisters on his feet, then callouses. He spends himself for the glory of God.
A good soldier ponders the Word of God, he mulls it over, chews on it, eats it so that he will bleed it when cut.
But a good soldier will keep foremost in his mind not his own wherewithal, gumption, or courage. That will all be sapped. In the chains of hardship, persecution, imprisonment, sin, or suffering, a good soldier will resolve to know nothing but Jesus Christ and him crucified.
Good soldiers, if you are flagging, dragging, or slacking, "Remember Jesus Christ, risen from the dead, the offspring of David" as preached in the gospel. As you strive, remember you are also seated with him in the heavenlies. And while you use your body up, perhaps even to the point of death in service to the Lord, your heart is expanding to fit the scale of eternity. The risen, glorious Christ shines in you, over you, before you, supplying his approval, his grace, his glorious might so that you will finish the race, reap the harvest, and receive the soldier's highest honor.
You, then, my child, be strengthened by the grace that is in Christ Jesus...
-- 2 Timothy 2:1
The body they may kill
God's truth abideth still
His kingdom is forever.
-- Luther, "A Mighty Fortress"
Photo credit: Soldier of US Army Taking Rest in Foxhole Near Front Lines Around Stolberg During Push Into Germany by John Florea