1 Corinthians 16:13

ἀνδρίζεσθε. Be manly, or behave like men. It is quite a mistake to insist exclusively on the softer characteristics of the Christian character. In the Christian, as in the soldier, endurance is of little use unless combined with courage. Courage is one of the most marked features of the character of Christ. ‘If you think Christianity a feeble, soft thing, ill-adapted to call out the manlier features of character, read here.’ Robertson.

κραταιοῦσθε. Grow strong, or be strengthened. The former is preferable, nor does it ignore the consideration which the latter makes prominent, that the source of our strength is not in ourselves, but in Christ. The classical form for κραταιοῦμαι is κρατύνομαι.