One hundred years ago this weekend, in a nondescript church in a township in Bloemfontein, tribal chiefs, religious figures and lawyers founded what would become the African National Congress. As white rule became more and more brutally oppressive, the ANC grew to lead, with others such as Steve Biko and Desmond Tutu, one of the great movements of resistance and liberation of modern times.

The movement was even greater than its greatest leader, Nelson Mandela. Consider Walter Sisulu. "He was the magnet that drew us all together," said Mandela. For his belief in non-racialism "he planned to meet the hangman with a song on his lips", eventually serving over 25 years in jail. Sisulu "helped me understand that my real vocation was to be a servant of the people," said Mandela. "Walter, as secretary general ... went out of his way to cultivate such a culture of vigorous debate, free of any trace of vindictiveness." Sisulu asked nothing for himself, and at the moment of triumph, in 1994, retired to his small red-brick house in Soweto and to quiet devotion to his wife.

Today's archetypal ANC official is the antithesis of Walter Sisulu, and since that moment of triumph the ANC has been a study in degeneration. Having brought South Africa one of the world's most progressive constitutions, the ANC is now working to subvert it. President Zuma is seeking a pliant judiciary and draconian secrecy laws, and lost no time abolishing the country's top anti-corruption agency. The ANC has brought improvements in services such as water, electricity and housing, its social grants have targeted the poorest, and income poverty has very slightly improved. But inequality has increased under the ANC, and is now driven by intra-black inequality. South Africa is one of the world's most unequal countries.

In the fight against HIV/Aids, the ANC has mostly been on the wrong side: a Harvard study concluded that its stance led to 365,000 early deaths. Once a moral beacon for the wider world, these days the ANC too often also seems on the wrong side abroad – both in its own back yard and in the security council. Peaceful protesters in Burma, Syria and Zimbabwe might have expected ANC empathy, but a misguided deference to sovereignty often leaves it in league with tyrants. Unlike the Dalai Lama, recently refused a visa, some of them will be wined and dined at the centenary celebrations.

The movement that once showed the world the triumph of the human spirit now offers a lesson in human frailty. This is high tragedy. But the ANC is not about to splinter or be voted out. There is still another chapter to be written, and it is high time for the ANC's many remaining idealists to start writing it.