Be not children in mind (μη παιδια γινεσθε ταις φρεσιν). "Cease becoming children in your intellects," as some of them evidently were. Cf. Hebrews 5:11-14 for a like complaint of intellectual dulness for being old babies.

In malice be ye babes (τη κακια νηπιαζετε).

Be men (τελειο γινεσθε). Keep on becoming adults in your minds. A noble and a needed command, pertinent today.