Proverbs 24:5 A wise man is strong; yea, a man of knowledge increases strength.
God promises spiritual strength, real strength, to those who believe in Christ and His Word. The one who believes in Jesus is a wise man and a man of knowledge. “The fear (faith) of the Lord is the beginning of knowledge,” and wisdom. It is not that it is a wise thing to believe in Jesus (although it is), but it is that the believer in Christ becomes wise and learns to “know” the Lord. I Timothy 3:15: “The sacred writings are able to make you wise for salvation through faith in Christ Jesus.”
The promise is that the believer who knows the Lord will be strong and will increase in strength. Only a smidgeon of faith, like a seed, is enough faith to save, for it is Jesus who saves you, not your faith that saves you. But that seed of faith can grow up into greater wisdom and deeper knowledge of God, like a tree planted by the waters. And it is that wisdom and knowledge that increases strength.
The promise is not for physical or political strength, but it is for inner spiritual strength. This kind of strength will grow and increase. This kind of mature Christian who has heard the Word of Christ over the years will be strong to overcome any trouble, trial, or testing that comes his way. This is the promise. “Be strong and courageous!” “The joy of the Lord is your strength.”