Why is the Presbyterian Cross invariably adorned with the symbol of a burning bush?
The name 'Presbyterian' comes from the Late Latin presbyter, which means 'an elder'. (The prefix pre- means 'before' or 'leader', and the Vulgar Latin gave us prester, meaning 'priest'.) The name implies that the Presbyterian Church is governed by elders rather than bishops.
John Knox (1505-1572) is credited with starting the first Presbyterian Kirk in Scotland. The Church of Scotland itself was eventually reformed along Presbyterian lines and Presbyterianism spread throughout the world. (This worldwide evangelism has had some success. Now 60% of Christians live in developing countries - more Presbyterians go to church in Ghana than in Scotland.)
The national Presbyterian Churches in each country have their own logo or emblem, and often they feature two things: a burning bush, and something to identify the country. In the case of the Church of Scotland (C of S), a Scottish St. Andrew's Cross is used as the background of their main emblem. An offshoot of the C of S is the Associate Reformed Presbyterian Church (ARP), which has an emblem showing a burning bush at the bottom left side of a Celtic Cross, an open Bible showing the Alpha and Omega symbols (ΑΩ) and also a dove.
In France, the Église Réformée de France (ERF) uses the burning bush and dove symbols found in the Huguenot Cross. The Free Presbyterian Church (FPC) of Ulster and other countries features a burning bush with the motto Ardens Sed Virens (Burning but not Consumed). The Presbyterian Church of Australia (PCA) has a cross superimposed on the Southern Cross of the Australian national flag. A dove tops of the seal of the Presbyterian Church (USA) (PCUSA), the red/white/blue colours match those of the Flag of the United States and the flames at the base represent the burning bush.
The burning bush is the feature of many other logos, including those of the national Presbyterian Churches in Canada and Taiwan.
The miraculous burning bush which was not consumed (Exodus 3) is a symbol repeated in Presbyterian Church emblems around the world and represents the bondage of the church in Egypt. Their emblems remind us that the church and its people, in every age and every culture, may suffer severe persecution. And yet God prevents His people from being destroyed. An alternative interpretation is that the fire of the Holy Spirit can burn away sin and leave the soul pure.
See also Burning Cross and Huguenot Cross.