The Old Commandment and the New Commandment (1 John 2:7-11).
Here John stresses the need to stand by what they have originally learned and experienced and by the Light that is already shining among them, and not to seek to those who claim to bring new light but only bring darkness, which is evidenced by their failure to love those who are true to Christ.
In interpreting these verses it is easy to jump ahead in the letter and then interpret this old commandment as the command to love one another. But in John 13:34 it is the ‘new' commandment that is to love one another, (as in fact here in 1 John 2:8), while the ‘old' commandment' is the offer of eternal life (John 12:50 compare John 14:31) which leads up to the new commandment, and encompasses the commandments as found constantly in the teaching of Jesus, which connect with this and are referred to constantly (John 14:15; John 14:21; John 14:23-24; John 15:10; John 15:14). It is those who hear His word and follow Him who receive eternal life (John 10:28). Thus while the new commandment is emphasised in John (as one commandment - John 13:34; John 15:12; John 15:17) His other words and commandments are also much emphasised. The disciples were to observe the whole width of His teaching, not just the commandment to love one another, however important that might be.
So we may see the principle commandment, ‘His commandment', as the offer of life eternal (John 12:50), which is received by hearing His voice and following Him (John 10:28). Compare 1 John 2:25. This is further proved in 1 John 3:23 where God's commandment is that we believe in the name of His Son Jesus Christ (and thus receive eternal life). That is then followed, importantly but secondarily, by a second commandment that we need to love one another in accordance with His commandment. So there we have two commandments also (1 John 3:24 a).