a soldier: a member of a fighting force who serves in the ranks under a commander's orders
Across the Gospels and Acts this is the ordinary soldier of the Roman world: the men who arrest, guard, flog, and crucify, and who cast lots for a condemned man's clothing at the foot of the cross. The word points not to a general or an officer but to the man in the ranks, the one who carries out orders whether or not he agrees with them.
Because a soldier's life was defined by obedience, hardship, and total commitment to his commander, the word became a natural picture of Christian discipleship. Paul tells Timothy to take his share of suffering as a good soldier of Christ Jesus. The figurative use keeps everything the literal one carried, the discipline, the cost, and the loyalty, and simply turns it toward a new master.