Finally, my brethren, be strong in the Lord, and in the power of his might.
Finally - after instruction to the church and individuals, Paul is wrapping up his letter with final thoughts.
We are not to find strength within ourselves and our abilities, but our strength comes from the Lord.
"But they that wait upon the Lord shall renew their strength; they shall mount up with wings as eagles; they shall run, and not be weary; and they shall walk, and not faint." (Isaiah 40:31)
"And he said unto me, My grace is sufficient for thee: for my strength is made perfect in weakness. Most gladly therefore will I rather glory in my infirmities, that the power of Christ may rest upon me. Therefore I take pleasure in infirmities, in reproaches, in necessities, in persecutions, in distresses for Christ's sake: for when I am weak, then am I strong." (2 Corinthians 12:9-10)
"That he would grant you, according to the riches of his glory, to be strengthened with might by his Spirit in the inner man;" (Ephesians 3:16)
"I can do all things through Christ which strengtheneth me." (Philippians 4:13)
"But the God of all grace, who hath called us unto his eternal glory by Christ Jesus, after that ye have suffered a while, make you perfect, stablish, strengthen, settle you." (1 Peter 5:10)
As long as we walk in our strength, we are but weak and powerless.