"Finally, be strong in the Lord and in his mighty power." NIV translation
The second lesson that a soldier learns in basic training is teamwork and trusting the man/woman next to them in the heat of the battle. In our spiritual boot camp, that teamwork is trusting in Jesus and His power to fight the battles with us. When we try to do things on our own, we will often fail but, with Christ, we are victorious! That is what John was talking about when he said that faith is the victory that allows us to overcome the world (see 1 John 5:4).
"Put on the full armor of God so that you can take your stand against the devil's schemes." NIV translation
Armor is used to protect the soldier during battle and the same is true for the Christian. The armor does not do any good unless it is put on the body and so Paul tells us to put it all on.
The word "schemes" is talking about the methods that the devil will use against man. We can be assured that, whatever the scheme, we will be protected with the armor that God has provided us.
"For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms." NIV translation
The third skill that we need to develop as a soldier is the ability to recognize the enemy. In this verse, Paul tells us who the enemy is and, as we see, it is not the people around us but the spirit of evil. This is talking about the devil and his demons. They are at work in this world and many times they work through people that don't even know it. It seems that, today, more people are wounded in their battles by the "church" than they are the devil. This is caused by the fact that many Christians have not developed this ability to recognize the enemy. The devil uses this "friendly fire" to spread division in hopes of slowing or even stopping the work of God here on earth.
"Therefore put on the full armor of God, so that when the day of evil comes, you may be able to stand your ground, and after you have done everything, to stand." NIV translation
Paul repeats how important it is to put on the "full armor" so that we can stand. God has given us everything that we need but we have to choose to put it on and to use it.