By 2050, Pew believes 45.1 per cent of New Zealand's population will be unaffiliated - making it the largest group in the country. Only France (44.1 per cent) and the Netherlands (49.1 per cent) saw a similar projection, the centre said in its Future of World Religions study.
But Dr Nick Thompson, lecturer in theology at the University of Auckland, believed this was a conservative estimate.
"I think it will be over 50 per cent well before the 2050 figure that the Pew people give," he said.
In the last two censuses, he said, there had been a more than 10 per cent jump in the number of people who identified as non-religious from 2006 to 2013. "It's pretty clear."
The only factor that would slow that increase would be immigration from countries where religious affiliation is a lot higher, Dr Thompson said.
"Especially among Pakeha, Maori and even to an extent Pacific Island communities, religious affiliation is either in decline or slowing down."
New Zealand had been "muddling along on a kind of cultural Christianity" since the '60s, Dr Thompson said.
"It may be that Christianity continues in the culture a long time after it's declined as a religion.
"When you think of Christmas, for example, it's a really, really popular festival, even though it would be hard to claim that the majority of people observing it were particularly religious.
"So the way we observe Christmas now may give you a sneak preview of the way we treat Christianity at the end of this century."
Mark Honeychurch, president of the Humanist Society of New Zealand, said the Pew projection was "not overly surprising. The only thing I was surprised at was that it would be up until 2050."