Recent developments on the Korean peninsula have inspired a degree of shock and awe in diplomatic circles from Washington to Beijing. South Koreans themselves have been mesmerized by the previously unimaginable spectacle of the smiling leaders of North and South Korea, Kim Jong-un and Moon Jae-in, and their hand-holding diplomacy across the Demilitarized Zone.

“It’s really astonishing for us to see what is happening these days,” Bishop Peter Kang U-il confirmed via e-mail. “We never imagined things would develop so fast.” Bishop Kang, a former president of the Catholic Bishops’ Conference of Korea, leads the diocese of Cheju.

Average Koreans may be gob-smacked by the unanticipated pace of the improving relations, but they hope follow-up negotiations and a much-anticipated meeting between U.S. President Trump and Kim Jong-un will keep the diplomatic momentum going, he said.

Developments on the Korean peninsula have inspired a degree of shock and awe in diplomatic circles.

Some in the United States give much of credit for the new opening on the peninsula to the Twitter-diplomacy (or more accurately lack thereof) conducted by Mr. Trump. It may be the only time denigrating another world leader as “rocket man” has led to a diplomatic coup. Some are even arguing that Mr. Trump should receive the Nobel Prize for Peace for—somehow—bringing the Korean leaders together. Bishop Kang had been critical of Mr. Trump’s approach to Mr. Kim in the past, which he considered unnecessarily provocative.

Now, he would only say, “Many Korean people have an impression that Mr. Trump is an unpredictable person, but they are also ready to bet on his rationality and common sense.”

As talks of a non-nuclear North and a trilateral peace treaty raise expectations, the skeptical in South Korea point out that Northern leaders have reneged before on promises of better behavior and the denuclearization of the peninsula.

Some are arguing that Mr. Trump should receive the Nobel Prize for Peace for bringing the Korean leaders together.

“However, many people seem to feel this time there have been fairly different signs in Kim Jong-un’s behavior and his way of expression,” Bishop Kang said. He cited a recent public opinion poll in South Korea that tracks a significant change of heart in the South regarding Mr. Kim.

Immediately after the historic meeting in the DMZ truce village of Panmunjom, 65 percent of South Koreans say they trust the sincerity of the North Korean leader regarding his desire for peace; almost half of even conservative Koreans say they accept Mr. Kim’s sincerity, according to the bishop. “It’s a radical change,” he said. “Most South Korean people don’t seem so suspicious about the intention of Mr. Kim, listening and watching his sincere attitude [during meetings with Mr. Moon].”

The church in the South will attempt to capitalize on the improving cross-border mood.

Mr. Kim’s decision to send “his own sister, his most trusted person,” to the Winter Olympic Games appears to have begun the warming trend among people in South Korea.

Bishop Kang said the church in the South will attempt to capitalize on the improving cross-border mood by continuing its outreach efforts to the North. It has been offering humanitarian assistance for more than two decades, he said.

What comes next for the Koreas? “The two leaders of the South and the North already pledged to work toward establishing a peace treaty to formally end the Korean War,” Bishop Kang said. Mr. Moon has even offered to include the North in a joint economic development program with the aim of accelerating trade with the European Union, Russia and China and “taking advantage of railroad connection” between the two Koreas. That possible integration “would certainly improve North Korea’s economy as well as the South and bring a lot of cultural exchange,” Bishop Kang said.

Is it all happening too fast? Bishop Kang does not think so. Most in the South “don’t seem nervous” about the sudden progress on peace. “They have waited so long for it,” he said. Even the skeptical young who had been “generally rather negative” to the idea of reunification with the North are now talking about their growing expectations for change on the peninsula—not least of which “the day when they would be free from [military] enlistment.”

Kevin Clarke is America’s chief correspondent and the author of Oscar Romero: Love Must Win Out (Liturgical Press).