‘If you really (or ‘however, if you') fulfil the royal law, according to the scripture, “You shall love your neighbour as yourself, you do well, but if you have respect of persons, you commit sin, being convicted by the law as transgressors.'
The basic principle here is simple. If they keep the law which says, ‘You shall love your neighbour as yourself' by treating all impartially they ‘do well'. But if they show partiality and reveal respect of particular persons such as the rich then they fail to keep that law, and they therefore become lawbreakers. They do ill. The law then convicts them of breaking the law, of being transgressors. This is even more emphasised by the fact that the law forbade showing discrimination against the poor (e.g. Leviticus 19:15).
‘The royal law (nomon --- basilikon).' The order of the words shows either that basilikon is accessory to nomon ("a law, a royal one"), or that it has a special force, it is a law which merits being called "royal." But the question is, why is it royal? One answer may be because it was pronounced by the King as the guiding law with respect to our attitudes towards our fellowman (Mark 12:31; Luke 10:27). Another may be that it is because it is the ‘king of laws'. It rules over and takes in all the others. Indeed both meanings may be intended by it, for they both merge. As Paul says, whoever fulfils this law has fulfilled the whole law, for ‘the whole law is fulfilled in this word, You shall love your neighbour as yourself' (Galatians 5:14). It is another way of saying, ‘do to others what you would have them do to you' (Matthew 7:12).
‘If you really fulfil --.' That is, in relation to what is in mind here. Thus it can be said of them that they ‘do well' as compared with those who show respect of persons. Had it meant that they fulfilled that law in all its aspects he would have said, ‘you have done amazingly well'. It was one of the two commandments, of which Jesus said that on them hung all the Law and the Prophets (Matthew 22:40). A royal law indeed!