There are a couple of things Paul says in Ephesians 6:10-18 that really ought to ring our bell. One is fairly obvious, but I think the other lies below the surface.
The truth we’re more familiar with in his emphasis on us relying on the Lord and His power (as opposed to our own). “Be strong in the Lord and in His mighty power,” we’re told in 6:10.
Now if we’ve been reading the Ephesians from the beginning, we would know that Paul has already had a lot to say about the Lord’s power. In 1:19, he has prayed that the eyes of our heart would be enlightened so they may know God’s “incomparably great power for us who believe.” We would know that this is the power that not only brought Jesus up from the dead, but seated Him at the right hand of God – “far above all rule and authority, power and dominion, and every title that can be given, not only in the present age but also in the one to come,” (v. 21). This same power has also raised us up from the dead and seated us with Christ in the heavenly realms (2:6).
Paul continues his prayer as he asks the Father to give us the power to grasp His love (3:18-19). And then there is “the power that is at work within us,” (3:21). This is the power by which God is able to do “immeasurably more than all we ask or imagine,” (3:20). Ephesians makes it clear—God has the power we need!
What Paul has to say in 6:10 is more of a conclusion—no, make that a crescendo, than a new thought. He saying, “In light of all that I’ve told you, take hold of the strength that is in the Lord. It is the only way you will be able to stand!”
Related to this is the second bell-ringer. As I’ve already mentioned, I think it is a more subtle truth relating to how we are to be strong in the Lord. Paul tells us a couple of times to “put on the full armor of God,” (v. 11, 13). He then details seven pieces of armor (if you count prayer as a piece). My understanding of this is not that we’re supposed to analyze each piece of armor and see it in some significant way. I think Paul is instead telling us the same thing in several different ways—cover yourself up in Christ! Whether it’s His salvation, His righteousness, His truth, His word, etc., we haven’t done everything to stand unless we are standing in Him! We must divest ourselves of any self-reliance and put our trust in Him (think of David refusing to wear Saul’s armor – 1 Samuel 17:38ff).
The picture Paul paints suggests that it’s not enough to be in the Lord’s army – we must put on the armor! If we haven’t done that, then we haven’t done what we need to do to stand. We’re in the army to wear the armor and fight the good fight, not so we can march in our parade dress. This is a call for our faith to be alive, active, and alert! Being casual about why we’re in the army could lead to us becoming a casualty! But we must not confuse being in the army with wearing the armor—they are not the same thing!