What Paul is saying is that you may not have been strong before you knew the will of God for your life, but now that you  know God's will,"from now on, be strong." The point however is not be randomly strong, but rather to be purposefully and intentionally strong. When Paul uses the word strong "dunamis" in the Greek, he is in fact saying be strong like "dynamite".

 

Take a moment to consider the difference between dynamite and nitroglycerine. Both are very powerful explosives. Nitroglycerine requires very gentle handling. If shaken in the wrong fashion it could explode unintentionally. Dynamite, on the other hand, must be intentionally detonated before it unleashes its true power. In other words, Paul says that from now on when the Devil confronts you detonate your "dynamite" against him.

 

Also,  there is a difference between God's power and God's might. God has unlimited might, which is a reservoir that never runs low and is always on display. God's power, on the other hand, is not on display until we tap into it. This is the reason some Christians live powerless lives. It is not that God lacks the might to change our situation, it is that we do not tap into God's power. When we "stand strong" in the Lord, we tap into the power of God's might.

 

Once we tap into the power of God's might, we can stand no matter what. In fact, the bible says that "after you have done everything in order to stand - keep standing."  When you don't see a way out in your might - stand in the power of God's might. In other words, don't let anything stop you.

 

Finally, God has design an armor to protect us from Demonic/Satanic onslaughts. Our amor includes salvation, the good news of Christ, and our Bible. When we put on the full armor of God, we are protected from all the weapons that Satan uses to attack us. Instead of running from Satan, we can take the word of God, use our "dunamis" our dynamite and run Satan out of our lives - out of our situations.