Rather Than Deserving Reproach And Calumniation They Are to Be The Salt of the Earth and the Light of the World (5:13-16).
Having commenced His sermon by revealing what the disciples are, by virtue of God's active work within them (His ‘blessing)', and having warned them against persecution as a consequence, in a similar way to the prophets, Jesus now explains the significance of it for them in the context of the world. They are present in the world in their new state as preserving salt and as revealing light. And although through it many will be blessed, that is why they will be persecuted (this is the story of Acts).
Here we have further evidence that His words in the Sermon are mainly directed to disciples. Not only are they to be persecuted for His sake, but their special influence in the world is to be powerful and all-pervading. And this could only be spoken of people in whom verses Matthew 5:3-9 have been actualised so that what is spoken of there has become a living experience.
We are aware, of course, of how far the disciples came short of this ideal, certainly for a long time, but at least a beginning has been made. They are now changed men, and on the way to becoming ‘perfect' (Matthew 5:48), fully matured in righteousness and love, even though they have a long way to go. They have therefore even now become agents through whom God will fulfil His purposes. This should comfort us with the thought that we too do not have to have become perfect before this can happen, for He reveals His glory in earthen vessels so that all the glory will go to Him (2 Corinthians 4:7).