Simon is described as marble after he is killed. Could this be symbolic of the laws of a civil society being an ideology and inherent in humanity?
I am trying to show Simon's role as a political symbol, not just spiritual.
Given that this is an English novel and that Golding's "Spire" directly addresses the structure of the Anglican Church, I think you may have a good starting point here.
The Church of England is a national church, of which the British monarch is the "governor," or secular head. Since the acts of praemunire, provisions, and provisors of the 13th century, and even more dramatically since the separation of the Church of England from Rome, the structure of the church has been defined as one in which the monarch has civil ministers (Cabinet, Prime Minister) and spiritual ministers (bishops and priests). Lord Bishops (ones with major dioceses) automatically sit in the House of Lords. (See Richard Hooker's "Ecclesiastical Polity" for the foundational structures of the Church of England).
This means that the religious and the political are not independent of one another. The religious leader is responsible for cure of the souls in the world, and that include intervening in politics -- e.g whether taxes are devoted to helping corporation or to the poor is a concern of religious leaders, as is the issue of whether war is justified. Many Archbishops of Canterbury have thus felt themselves called to comment on "political" matters.
Thus with Simon, you might want to argue that the separation of political from religious does not in fact reflect the Anglican theological background to the novel.
For marble, think also of the graves in Westminster Abbey with marble statues of their occupants. The marble is more durable than flesh. (The pieta statues, with the dead Christ in Mary's lap is important as well) Does Simon's influence transcend his death?