After Muhammad died, the Muslim leadership was assumed by Abu Bakr. He was followed by Umar, Uthman ibn Affan and then Ali. These four are called the "Rashidun caliphs" which means The Righteous Caliphs. The word caliph means religious and spiritual leader. Under these caliphs Islam became one of the most important religions of the middle East. Administrative offices of the Muslim nation were established. Under Umar more than two thirds of the Eastern Roman Empire was conquered by the Muslims. Umar also started the Muslim calendar.