Somewhat more than half Sudan's population was Muslim in the early 1990s. Most Muslims, perhaps 90 percent, lived in the north, where they constituted 75 percent or more of the population. Data on Christians was less reliable; estimates ranged from 4 to 10 percent of the population. At least one-third of the Sudanese were still attached to the indigenous religions of their forebears. Most Christian Sudanese and adherents of local religious systems lived in southern Sudan. Islam had made inroads into the south, but more through the need to know Arabic than a profound belief in the tenets of the Quran. The SPLM, which in 1991 controlled most of southern Sudan, opposed the imposition of the sharia (Islamic law).