In the Summa Theologia, the second question on the existence of God is headed, 'An Deus sit.' Here we have to be careful about how we translate this. It does not mean, 'whether there is a God'. It should be translated as, 'whether there is God'. To say 'a God', with or without capital letters is to have gone wrong from the start. It is not just that God cannot be seen as one of a possible range of beings, which the indefinite article implies. To ask the question, 'Is there God' is to ask about reality as a whole. God is part of the proper description of all reality. To say that God exists is to make a statement about all that there is. Everything depends for its existence on that which does not depend for its existence on anything, in any way. 'There is God' is a statement that says that all reality has an explanation, that all reality is a choice, made by the one who cannot be subject the choices of others but who has allowed us to choose for ourselves.
The statement 'God does not exist' is not a statement about all reality. It is merely the preliminary statement for some other statement about reality, though I am not sure what that statement would be. Many modern atheists compare belief in God to belief in Gods, but in point of fact if the statement 'There is God' is true, then it is impossible for there to be Gods. There cannot be Gods which just happen to exist. Everything including immensely powerful beings, which God might have caused to exist, without being part of a process of previous causes, would still depend on God for their existence.
The people of Israel were the people of the God who is. They learned that their God was not 'a god'. They learned that to turn away from God was to turn to other Gods, the God who are not. They were the light of the world, and the salt of the earth. It is Israel first whom Our Lord addresses in the Sermon on the Mount, and from them, all Christians in the new Israel, in the revelation of God which goes beyond human reason. Light and salt may seem quite different things, but they have this in common. Light is invisible in itself but becomes visible by the things it illumines, and salt does not so much have flavour, but rather brings out the flavour of other things. So light has to be the light of something, salt has to be the salt of something. So Our Lord speaks of the light of the World, the salt of the Earth, that is to say, all the reality in which we live.
Salt is used as a metaphor for wit, and St Paul in Colossians 4:5-6 says that we should 'Walk in wisdom toward outsiders, making the best use of the time. Let your speech always be gracious, seasoned with salt, so that you may know how you ought to answer each person.' Yet Our Lord is seeking something more than cleverness or eloquence from us. We are to let our good works be known. In fact, he doesn't say good works but beautiful works. The works that Christians sometimes manage to carry out by God's grace have a strange sort of beauty about them. That which is truly beautiful is often only perceived in its beauty after much thought and study. The beautiful works of which Christ speaks are the works of the Beatitudes which precedes this passage and govern the whole of the Sermon on the Mount of which today's Gospel is part. These works find their ultimate perfection in that most paradoxical of beautiful works, the death of Christ on the cross. If we are to be light and salt for the world, we must be part of that beauty. This beauty seems like ugliness, just as the happiness of the beatitudes seems like a contradiction of what we mean by happiness. Yet this beauty is the truth of the beauty of creation, a truth which unfolds day by day, until He comes.