RSV Commentary

Finally, my brethren, be strong in the Lord, and in the power of His might (v.10). This word finally is literally for the rest. This is not necessarily Paul finishing this letter, but to say the last thing I want to leave you is this, and he continues with his instruction. Paul has done this in his letter to the Corinthians and to the Philippians, meaning he is not ending his letter but giving final instructions.

Brethren are believers and Paul is telling them to be strong in the Lord. Be strong in the Lord is to be strengthened in Him. This is a command and it is a continuous action, meaning it is something that believers need to constantly be doing. Our strength comes from the Lord Jesus Christ. Paul has already prayed that they would be strengthened in Ephesians 1:19 and 3:16. One was for them to know the power of God and the other is to be strengthened by His Spirit. So, basically, it is to be enabled by strength in the Lord.

(Eph. 6:10 KJV)