Albert Barnes on Ephesians 6:10

Finally, my brethren, be strong in the Lord. Paul had now stated to the Ephesians the duties which they were to perform. He had considered the various relations of life which they sustained, and the obligations resulting from them. He was not unaware that, in the discharge of their duties, they would need strength from above. He knew that they had great and mighty foes, and that to meet them they needed to be clothed in the panoply of the Christian soldier. He closes, therefore, by exhorting them to put on all the strength which they could to meet the enemies with which they had to contend; and in the commencement of his exhortation he reminds them that it was only by the strength of the Lord that they could hope for victory. To be "strong in the Lord," is,

(1.) to be strong or courageous in his cause;

(2.) to feel that he is our strength, and to rely on him and his promises.

(*) "might" "in his mighty power"