Brethren begins a new paragraph. It refers to “fellow Christians,” not actual family members. The connection with what precedes this word is not entirely clear, but Paul appears to mean that the Corinthian Christians were “immature” in giving too much importance to speaking in tongues. This first sentence may be rephrased as “Fellow Christians, do not think like children” or “… like immature people.”

The verb translated be babes is not related in form to children in the first part of the verse, but the meaning is the same. As elsewhere, Paul, a good writer, varies his language where possible. This sentence may be translated in some languages as “you must be innocent like babies in the ways of evil.”

The word for evil refers to general activity that is wrong or sinful, rather than to evil persons or things. The sentence be babes in evil may be rendered in some languages as “You must be innocent like children in the ways of evil” or “… in not knowing evil doings.”

Mature or “grown up” (Good News Bible) was translated “spiritually mature” (Good News Bible) in 2.6, and that is what mature means here. If verse 20b stood alone, it could be translated “become grown up…,” but this translation is not possible in verse 20a, so be mature is correct.

The whole verse may thus be translated: “My brothers, do not think in a spiritually immature way; you must be innocent like children, as far as evil is concerned, but think as people who are spiritually mature.”

Quoted with permission from Ellingworth, Paul and Hatton, Howard A. A Handbook on Paul’s First Letter to the Corinthians, 2nd edition. (UBS Handbook Series). New York: UBS, 1985/1994. For this and other handbooks for translators see here .