Mr. Kim appeared in a government-arranged news conference in Pyongyang in February, calling himself a “criminal” and apologizing for the “anti-state” crimes he said he committed against the North while working at the behest of the South Korean spy agency.

Although Seoul denied using missionaries against the North, Pyongyang has said the South’s National Intelligence Service was behind the espionage to undermine its political system through “psychological warfare.”

South Korea has demanded Mr. Kim’s release but the North has not responded. The rival governments on the divided Korean Peninsula do not recognize each other, agreeing to dialogue only when both sides are in a conciliatory mood.

North Korea has often been often suspected of using people held in the country as political bargaining chips to force Washington and others to negotiate with it. It had previously released United States citizens it had held when prominent Americans, such as the former President Bill Clinton, visited the country.

But the North has also been using the case of Mr. Bae and Mr. Kim as a warning to other South Korean or Korean-American missionaries and Christian activists who have been active along the Chinese-North Korean border for years.

The missionaries and activists distribute food for North Korean refugees and sometimes help smuggle them for eventual resettlement in South Korea. But some of them have also recruited refugees and trained them as missionaries, sometimes dispatching them back into their home country to spread the Gospel and open underground churches — which could cost them their lives if they are caught.

Officially North Korea says it guarantees religious freedom. Under its new leader, Kim Jong-un, however, it has vowed to step up its efforts to block harmful influences from the outside, especially Christian proselytizing. Earlier this year, an Australian missionary named John Short was arrested in Pyongyang for allegedly trying to distribute Christian materials. He was later expelled after he apologized.

During Mr. Kim’s trial, prosecutors called for a death sentence, presenting “evidence such as religious books, memory cards, sex CDs and spying devices carried by the accused for criminal purposes.” But his lawyers asked for leniency, saying that he “sincerely repented of his crimes and apologized for them.”