NAIROBI, KENYA — NIGERIA, home to nearly 1 out of 5 Africans, has taken another big step toward civilian rule by late 1992.As results of the Dec. 14 gubernatorial elections in all 30 states were tallied, Nigerian analysts were interpreting the fact that the elections came off smoothly as evidence that the transition from military to civilian rule is continuing. Nigerian Maj. Gen. Ibrahim Babangida has promised to step down from the presidency by late 1992 - provided that the two-party civilian system the military has set up is still working and has chosen a president. The Dec. 14 elections were a key test of the power-transfer to civilians. After the election, Frank Aigbogun, editor of the independent Nigerian newspaper Vanguard, told Reuters: "There is no doubt the NRC [National Republic Convention - one of the two parties] has become a national party and should be able to win the presidency." The more liberal Social Democratic Party suffered some losses because of splits in its ranks. Near-final results gave the NRC an advantage of several governorships. But observers are encouraged not so much by which party is emerging strongest, but that both parties appear to be functioning well enough to mount national campaigns. To clean up politics, the military banned past politicians and current military leaders from running for office before the end of 1992. Nigeria has known civilian and military governments, but none has satisfied public demands for civil liberties, accountability, and efficiency. There are serious challenges ahead. Religious tensions, including a rivalry between the mostly Muslim north and the mostly Christian south, often threaten to tear the country apart. On a September visit to Nigeria, this correspondent stood in the ashes of a Christian church in Zaria, northern Nigeria, burned down by Muslim zealots after a Christian made a disparaging remark about Islam. Asked about tribal rivalries, L. S. Aminu at the Nigerian Institute of International Affairs in Lagos, Nigeria's capital, was optimistic. Ethnic divisions appear to be diminishing, he told the Monitor in September. And ethnic issues are "not selling" in election campaigns, he said. As for the transition process, he added: "I think we're getting somewhere. A new crop, or generation [of political leaders] is coming up." And both parties, he said, "are becoming national parties." If Nigerians can keep these tensions in check, and hold an orderly, fair presidential election late next year, Nigeria soon may have civilian government. Otherwise, Nigerians may see military rule for longer than most of them want.