The exodus happened after The Church of Norway enabled a new online option Aug. 12 that allows citizen to join and leave the church electronically.
More than 10,000 people deregistered within 24 hours of the site's launch, and within four days, more than 15,000 had left, the Independent reported.
"The number of withdrawals must be seen in relation to the large number of members of the Norwegian Church," Helga Haugland Byfuglien, head of the Norwegian Bishops' Conference, said in a statement on the church's website. "We have great respect for individual choice."
About 1,200 new members joined online in the same time period.
"No one who doesn't wish to be a member of the Church of Norway should be registered as a member," said Kristin Gunleiksrud Raaum, leader of the church's national council. "I'm very happy that almost 1,300 chose to join in August."
The electronic offering was put in place as a way for the church to get its "records in shape and offer an easy way for people to sign up," the Independent reported.
"We will continue to have a broad and open national church. But no one should be a member of a religious community against their will, and therefore I am glad that this self-solution is in place," Raaum said. "Those who mistakenly listed as a member of the Norwegian Church or who do not wish to be members can now easily change their status, and it will give us a more accurate registry."
About 73 percent of the population counted as members of the Church of Norway in 2015, according to the AP, but a significant portion of those people may have been among those who deregistered this year.
The AP reported that the country is among the most secular countries in the world, and a recent survey of 4,000 Norwegians showed this year that non-believers outnumber religious people with 39 percent saying they didn't belive in God, 37 percent saying they did and an additional 23 percent of respondents saying they did not know, The Local reported.
Norway, which previously required at least half of all government ministers to be members of the Church, did away with the Evangelical Lutheran religion as the official state religion in 2012, but the country maintains a constitution built upon "Christian and humanistic heritage," and the king is required to be Lutheran.