Geneva was a border town, fortified against the Celtic tribe Helvetii, when the Romans took it in 121 BC. It became Christian under the Late Roman Empire, and acquired its first bishop at this time. In the Middle Ages, Geneva was ruled by a count under the Holy Roman Empire until the late 14th century, when it was granted a charter giving it a high degree of self-governance. Around this time the House of Savoy came to (at least nominally) dominate the city. In the 15th century, an oligarchic republican government emerged with the creation of the Grand Council. In the first half of the 16th century, the Protestant Reformation reached the city, causing religious strife during which Savoy rule was thrown off and Geneva flirted with joining the Swiss Federation. In 1536, with Protestantism in the ascendancy, John Calvin, the founder of Calvinism, became the spiritual leader of the city. By the 18th century, however, Geneva had come under the influence of Catholic France, which cultivated the city elite, who tended to be at odds with the ordinary townsfolk – to the point that an abortive revolution took place in 1782. In 1798, revolutionary France under the Directory annexed Geneva. At the end of the Napoleonic Wars, in 1815, Geneva was admitted to the Swiss Confederation. In 1907, the separation of Church and State was adopted. Geneva flourished in the 19th and 20th centuries, becoming the seat of many international organizations