I will answer the 2nd question first. It is a matter of convention really and sensitivity to other faiths. Although the root of the concept was originally faith based – Rerum Novarum, the Catholic Pope Leo XIII encyclical on “Rights and Duties of Capital and Labour” – the ideology itself is not exclusively Christian (other encyclicals followed; Pius XI’s Quadragesimo Anno, John XXIII’s Mater et Magistra, etc). Elements of Islam and the other faiths actually define the ideology. Social justice, very much a core of the encyclical, transcends religion. Instead of Christian, Centrist was introduced – which brings me to the 1st question asked.

The encyclical focused on the relationship between labour and capital – supporting the rights of labor to form unions, rejecting the two extremes of communism and unrestricted capitalism while affirming the right to private property.

This middle ground or “center” became the expanse within which this emerging system of beliefs was ensconced.

But a more interesting and in fact romantic notion was the explanation of the Political Spectrum vis-à-vis the French Parliament just before the French Revolution – on the seating arrangements of its members.

To the left of the Speaker are the commoners and to the right were the aristocracy and the church. Convention later dictated that the left protected the interest of the Bourgeoisie and the right, the interest of the capitalist.

Thus the Political Spectrum is a way of comparing or visualizing different political positions by placing them upon one or more geometrical access.