We need to examine the word strong that is used here in the Greek so we can get a better understanding of what Paul is saying here. Endunamoo - to empower, enable, strengthen, make strong, be strong. As Paul is getting ready to explain the spiritual armor that we need to have on, while he was in prison with Roman guards all around him. So he saw what they did to strengthen themselves not just physically but also mentally and emotionally.
It is important that we know about what God has done in the past. Most of all remember what He has done in our lives. By knowing and remembering Him we will be empowered not only in trials and temptations but also to share your testimony to empower someone else. We have an enemy that will seek to destroy us physically, emotionally and spiritually. Knowing who God is and what He has done gives us strength to stand and fight back. Paul didn't see himself as in captivity, instead he had a captive audience. He shared the Gospel with the guards that were with him and many of them came to know Jesus as their Savior.
In order to know what God has done in the past we need to be in the Bible daily that way we know and are empowered by what He has done, can do and says He will do. Another thing is to write our own story of what He has done for us. Take the time and write each day how He has worked in your life that day. Then you can go back in hard times and see how He has been faithful in the past, so you have that to draw strength from for that time.
I always love when music is able to show me a new way to see God. Since Paul is preparing us for spiritual warfare, it seemed really appropriate that we remember that our Commander is a Mighty Warrior.