In some ways, it seems a strange command. It's easy to imagine that strength is something we don't have any control over; some people are strong and others are weak, just as some people are tall and others are short. But that's not true physically and it's not true spiritually either. Physically, we become stronger by intentionally exercising our arm muscles, leg muscles, heart rhythm, lung power or whatever it is we want to strengthen. God would never command us to do something that's impossible for us. So when Paul tells the early believers to be strong in the Lord, it implies that there's something we can do about that: we can respond in a way that intentionally helps us become spiritually stronger and more able to draw on the supernatural power of God.
In its Biblical context, the writer goes on to unpack this (verses 11 and 13) by telling the believers to put on every single piece of God's armour…. with the result that we will be able to resist the enemy. And, even if we happen to face battles and have a hard time, the ending will be that we are still standing firm. The Bible passage about the armour is well known, but it doesn't specifically tell us what to do to use that armour, and so there's a danger that we could leave it at a theoretical level and not actually take any of the steps needed to help ourselves be strong in the Lord and in His mighty power.
Just reading the Bible passage about putting on the armour will not make us spiritually strong, any more than reading health magazines will make us physically strong if we don't actually do anything in response to what we read. We need to find ways of taking these six spiritual weapons out of the realm of theory and into the realm of our daily reality and experience, or we won't see any difference, any tangible result in our own lives.
On Christmas Eve, my message in the church service was about six miles and six gifts. This morning, my preaching is about those six weapons, and how to use them practically in 2017 so that we end the year having become stronger in the Lord.

