1Corinthians 14:20-21 Brethren, be not children in understanding: howbeit in malice be you children, but in understanding be men. In the law it is written, With men of other tongues and other lips will I speak unto this people; and yet for all that will they not hear me, said the Lord.
‘Be not children in understanding: howbeit in malice be you children, but in understanding be men.’ Three stages of human growth are mentioned:  The Greek word nepios, meaning an infant who cannot yet speak and who knows nothing of sin (the verb nepiazo is translated “be you children” in 1Corinthians 14:20).  The Greek word paidion, a child beginning schooling to receive their first instructions.  The Greek word teleios, a man of mature age and thought; a man of growth and understanding. In other words, don’t be as little children in understanding. In malice be infants who cannot speak and who knows nothing of sin, but in understanding be people of maturity and growth.
‘Law’ – the law was a term used by Jews to express the whole Scriptures of the law, the prophets, and the psalms (Luke 24:44; John 10:34; 15:25), which we call the Old Testament today. ‘…written, With men of other tongues and other lips will I speak unto this people; and yet for all that will they not hear me, saith the Lord.’ This is the 5th and last Old Testament prophecy fulfilled in 1 Corinthians (Isa. 28:11-12). This prophecy reveals that God intended over 700 years before Christ to speak to people with stammering lips and other languages.