In Chapter 2 of 2 Timothy Paul lays out an amazing word picture of the different types believers and focused upon Pastors. They are a Son, Soldier, Athlete, Farmer, Workman, Vessel and Servant. Paul shows us from these windows how the Christian is to live and respond to the world and the evil there of.
Paul was no stranger to being jailed or imprison, matter a fact, Paul would have spent a great deal of time around the Roman soldiers. He probably would have gotten to know some of them very well. Speaking to them sharing the gospel to them, and them sharing their training regiment would have possibly given Paul the inspiration to write this as he did regarding being a soldier for Christ.
Paul knew very well that Christians are in a battle. “ Spiritual battle” If you do not come ready, you will be rendered ineffective for the kingdom sake. You will be useless because you were not ready for battle.