The history of religion in the Netherlands has been characterized by considerable diversity of religious thought and practice. From 1600 until the second half of the 20th century, the North and West had embraced the Protestant Reformation and were Calvinist. The southeast was predominately Catholic. Associated with immigration from Arab world (North Africa and the Middle east) of the 20th century, Muslims and other minority religions were concentrated in ethnic neighborhoods in the cities. Since the 1960s, the Netherlands has become one of the most secular countries in the Western world. In a December 2014 survey by VU Amsterdam, more atheists (25%) were reported than theists (17%) for the first time in the history of the Netherlands. The majority of the remainder of the population identified as agnostic (31%) or ietsist (27%). Before the advent of Christianity, the Netherlands were populated by Celtic tribes in the South, which adhered to Celtic polytheism, and Germanic tribes in the North, which adhered to Germanic paganism. After the Roman Empire occupied the later southern Netherlands, Roman mythology became important there, as well as religions from the Middle East, including relics from Egyptian mythology, Judaism, Mithraism and later Christianity. The oldest data on the profession of religion by the inhabitants of the regions that are now the "Netherlands" were passed down by the Romans. Contrary to what ancient sources seem to suggest, the Rhine, which clearly formed the boundary of the Roman Empire, did not form the boundary between residential areas of Celts and Germans. Germans settled south of it (Germani Cisrhenani), and many place names and archaeological finds indicate the presence of Celts north of the Rhine. Between these "Celtic – Germanic peoples" and later the Roman conquerors (romanization) a cultural exchange took place. An adaptation of polytheistic religions and other myths occurred among the various tribes, absorbing influences from Germanic, Celtic and later Roman mythology.