‘But concerning love of brothers and sisters you have no need that one write to you, for you yourselves are taught of God to love one another.'
Note the strong contrast between the fornication of the previous verses based on sexual love and the true Christian love here. They are as far apart as black from white. ‘Love of brothers and sisters' is ‘philadelphia'. It is a love based on the idea of love between blood brothers. It has no sexual connotations but stresses loyalty and desire for the wellbeing of the other. It now applies to all Christians because they are brothers and sisters.
‘For you yourselves are taught of God to love one another.' It was taught in the Law (Leviticus 19:18; Galatians 5:14), it was stressed by Jesus (Mark 12:31; John 13:34-35; John 15:12-13; John 15:17), it is the fruit of the Spirit (Galatians 5:22). It is central to the Christian message. Here the word for love is agape, the higher and nobler form of love that transcends feelings (see 1 Corinthians 13:4-8), given a new content by the Christian message. To behave towards others as you would have them behave towards you (without misapplying the idea), is to reveal such love. It is the untainted love of comrades-in-arms, which true Christians engaged in spiritual warfare feel instinctively when they meet other Christians engaged in the same.
‘Taught of God.' Literally ‘you are taught of God ones'. This links with the idea of the Holy Spirit being within them. True Christians are ‘taught of God ones' and it will reveal itself in their lives (compare 1 Corinthians 2:12).