‘Beloved, no new commandment do I write to you, but an old commandment which you (plural) had from the beginning. The old commandment is the word which you heard.'
Having described them as his ‘little children' John now addresses them as his ‘beloved ones'. He exemplifies in himself the truth he is exhorting them to. And he wants it immediately clear that he is not bringing to them something new, but something that they had ‘from the beginning', from when they first responded to Christ. Others may come with innovations but he will bring to them only the true word which was spoken by Jesus and which they received when they were first converted and which has been responsible for all their blessing.
And what is that old commandment? It is what God commanded. Jesus said, ‘for I did not speak from Myself, but the Father Who sent me, He has given me a commandment, what I should say and what I should speak, and I know that His commandment is life eternal. The things therefore which I speak, even as the Father has said to Me, thus I speak' (John 12:49-50). So it is what He told them. It is ‘the word that they heard'. And what is that word? It is the commandment of eternal life (John 12:50). It is concerning the Word of Life (1 John 1:1). It is His word through Jesus.
It includes the wide scope of the teaching of Jesus seen as included in one commandment, the commandment of eternal life. It is that they must look to Christ as the light of the world, the light of life (John 8:12). It is the word of the cross, that Christ crucified is the power of God to salvation through His work on the cross and through His resurrection (1 Corinthians 1:18 compare 1Jn 1:7; 1 John 2:2; John 6:52-59; Mark 10:45). It is that sin must be abhorred (1 John 1:7-10). It is that they must keep His word and His commandments, His teaching (John 14:10; John 14:23 with John 14:15; John 14:21; John 15:7 with John 15:10) as those who enjoy eternal life. It may be seen as including that they must love one another, although that is the emphasis of a new commandment (John 13:34; John 15:12; John 15:17). Thus they are to look back to the old foundations that they first received in the traditions about Jesus. Compare here 1 John 3:10 where doing righteousness (which includes loving God and one's neighbour) and loving one's brother are two major aspects of the Christian life.