Kratai-osthe. There are just a handful of occurrences of the word “be strong” in the New Testament:
And the child grew and became strong in spirit, and he was in the wilderness until the day of his public appearance to Israel. Luke 1:80
Be watchful, stand firm in the faith, act like men, be strong. 1 Cor. 16:13
that according to the riches of his glory he may grant you to be strengthened with power through his Spirit in your inner being, Eph. 3:16
Of course this based on the word for strong (kratos) which has many more occurances:
He has shown strength with his arm; he has scattered the proud in the thoughts of their hearts; Luke 1:51[He] alone has immortality, who dwells in unapproachable light, whom no one has ever seen or can see. To him be honor and eternal dominion. Amen. 1 Tim. 6:16
…and made us a kingdom, priests to his God and Father, to him be glory and dominion forever and ever. Amen. Rev. 1:6
What is particularly unique about this word is that it’s root word is kratos which was the name of a Greek god, the god of strength.
2901 From krataios; to empower, i.e. (passively) increase in vigor — be strengthened, be (wax) strong.
Helps Word Studies
2901 krataióō (from 2904 /krátos) – to prevail by God’s dominating strength, i.e. as His power prevails over opposition (gains mastery). See 2904 (kratos). For the believer, 2901/krataióō (“attain mastery, the upper-hand”) operates by the Lord inworking faith (His persuasion, 4102 /pístis).
It is related to, naturally, the word kratos, or power.
2904 kratos: strength, might
Helps Word Studies
2904 krátos (from a root meaning “to perfect, complete,” so Curtius, Thayer) – properly, dominion, exerted power.
The ancient Greek context fleshes it out even more vividly:
I get the impression that this word represented not a mere attribute of our inner being but a real and visible presence of something. In Luke’s gospel, he notes that Jesus grew strong in his spirit. But it must have somehow been visible for people to have recognized it, otherwise how could anyone, much less Luke, know? How can we identify this in men who are following Christ?
Be strong: have a mastery over the world.
Be strong: prevail over evil.
Be strong: dominate the temptation to sin.
The Apostle Paul doesn’t seem to be exhorting us to less than that.