In chapter 6 of Ephesians Paul reminds the church that there's an enemy to all that he just shared. He knows when these principles and promises are known, believed and lived out by faith, they will be life and joy to everyone who walks by them and is a witness to them. He also knows they are a threat to the enemies agenda... and that the enemy is out to dilute and water them down.
Seeing the enemy longs to deceive you, that He may destroy you, he uses things to distract, distort, disrupt, deflate and derail your ideas and thoughts on the will of God, His gospel, Spirit filled living, kingdom marriages, biblical submission, honor, obedience, long life, service, business and leading others for the glory of God.
When he says, "Be strong in Lord and in the power of His might..." I believe what Paul is saying, is this, "Be WISE [in] the Lord and in the MEANS [of] His might... and the "might" he's speaking of is the might and stability that comes from His knowledge, understanding and discernment. There's plenty of other truths to support this thought... but one the clearest indicators is the way he explains the method and gauntlet of attacks that the the enemy will us to distract and deceive.