"Thou therefore endure hardness, as a good soldier of Jesus Christ. No man that warreth entangleth himself with the affairs of this life; that he may please him who hath chosen him to be a soldier." (2 Timothy 2:3-4)
From a kingdom perspective, a good soldier has several responsibilities. Initially, we can expect challenges, wherein we might "suffer trouble as an evil doer" (2 Timothy 2:9), endure afflictions (2 Timothy 4:5), or even be afflicted (James 5:13).
Ultimately, a soldier has one purpose, "that he may please him who hath chosen him to be a soldier." Put another way, "do I now persuade men, or God? or do I seek to please men? for if I yet pleased men, I should not be the servant of Christ" (Galatians 1:10). Soldiers are called out of the normal life of a nation and dedicated to executing the will of the King.
Thus, from a spiritual perspective, "know ye not that friendship of the world is enmity with God? whosoever therefore will be a friend of the world is the enemy of God" (James 4:4). The source of that friendship is a focus on walking by the flesh, which has no good thing in it and cannot please God (Romans 8:8).
We are to "war a good warfare" (1 Timothy 1:18) and to "fight the good fight of faith" (1 Timothy 6:12) because "we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places" (Ephesians 6:12).
Fighting God's battles with God's armor insures the ultimate victory promised by our King, Creator, and "captain of the host of the LORD" (Joshua 5:14). "He will swallow up death in victory; and the Lord God will wipe away tears from off all faces; and the rebuke of his people shall he take away from off all the earth: for the LORD hath spoken it" (Isaiah 25:8). HMM III