"many Afrikaners were calling themselves Doppers, after the little metal caps with which they snuffed out candles"

The Afrikaners followed the Calvinism of the Dutch church.  However, as the Dutch Church underwent reforms associated with the Enlightenment, in the late 1700s, the church in both the Netherlands and South Africa experienced a number of schisms.

Bust of the Reverend Dirk Postma
Public DomainBust of the Reverend Dirk Postma

The Doppers were a particularly conservative faction who broke away from the South African Dutch Reformed Church (the Boer Nederduitsch Hervormde Kerk), the state church of the South African Republic. Under the Reverend Dirk Postma about 300 congregants formed the Gereformeerde Kerk in Rustenburg in February 1859.

The origin of the name Dopper is not certain, but may come from the Dutch domp (wick-snuffers) for their opposition to candles in worship, or from the Dutch word dop (to drink), for their opposition to small, individual communion cups.  The Doppers followed a severe puritan doctrine, and adopted distinctive dress and speech.  The men wore a short single-breasted coat, loose trousers, and broad-brimmed hats. 

They were strongly opposed to all English influence on their culture.  They were also opposed to all new inventions and mechanical contrivances that had not been known or used by their ancestors.  WW Collins described the Doppers as 'possessed with the idea that they too are a Divinely favoured people in the same sense that Israel was.'

Despite the small size of the community, they were disproportionately influential during and after the Great Trek. One of the most famous members of the Dopper church was the first President of the South African Republic, Paul Kruger.