"Be strong ..." is easier said than done.  It comes after Paul's instruction about how to live as Christians in the church (Ephesians 4:1-5:20), marriage (Ephesians 5:21-33), the family (Ephesians 6:1-4) and at work (Ephesians 6:5-9).  All those relationships are very challenging and need wisdom and strength.   But what kind of strength and why?  In the most difficult of battles, there is no physical enemy to stare-out, or physical battle to fight.  Our opponent is the devil and his agents: the instruction is about being strong 'in the Lord' and not 'in ourselves'.  The reason is that the Lord's Enemy hates the Lord's people, and will do whatever He can to stop their faithful discipleship.  

To be 'strong in the Lord' implies, and really means, that we are weak in ourselves and need to hide inside the Lord's protection.  As the next few verses will unpack, the believer is a special target for invisible but intelligent evil beings.  They are committed to suppling misinformation (lies) to our minds – tempting us to believe that evil is good for us - and terror to our hearts.  A common lie is that we can handle everything on our own, therefore we do not need the wisdom or protection of God.  The humanistic solution 'that if only we have enough knowledge and help, humans can surmount every problem' sounds attractive, but by excluding reliance on God, it leaves us weaker than we think and dangerously unprotected.

Because we live in a world which is spiritually dominated by unseen evil, the believer needs God's wisdom, energy and integrity to resist the devil, and all his demonic incursions into our lives.  The strength we need can only come from God Himself.  At school, a bullied child feels much stronger when he calls his big brother to sort out the bully.  The child has no more strength than before, but when he trusts his big brother to sort out the bully, he is no longer an intimidated weakling.  Using that picture, the Lord is our big brother (Hebrews 2:11).

The Lord's ability to deal with everything is infinite and absolute. He has absolute authority over demons (Mark 1:23-28).  Trusting Him to exercise His power around us in the church, the family and at work is enormously strengthening.  Instead of cowering in fear we can stand up in the strength that enables us to stand firm, while the Lord sorts out the enemies.  There is no other way to live the Christian life.  Apart from Him we can do nothing (John 15:5).  Without a confident trust in His ability to act in our hearts and in our circumstances, we will always be weak.  But the person who trusts the Lord for both the solution and the power to reach it ... will find their own strength renewed, with a fresh ability to keep going until the job is done (Isaiah 40:28-31), in His strength.