BEIJING—Christian groups who flouted a Chinese ban on foreign missionaries are calling their underground evangelizing during the Olympic Games a success.

Drawn to a nation of 1.3 billion people under atheist rule, the groups prepared for years for what the Southern Baptists once called “a spiritual harvest unlike any other.”

“We did see some conversions,” said Christian missionary Mark Taylor of Pensacola, Fla.

For Taylor, planning began four years ago with a lunch at the Athens Games among his Florida-based Awaken Generation ministry and ones from other countries. In the ensuing years, they came to China as tourists, making contacts among local Chinese.

Taylor—who left China on Friday—said 115 people from 12 countries gathered in Thailand for orientation before scattering throughout China, from Tibet through the far northeast. Two groups worked in Beijing, he said, though he would not give details.

Other larger efforts were carried out by the U.S.-based Southern Baptist Convention and the international ministry Youth With A Mission, Christian groups told The Associated Press. Neither ministry could say how many people were sent in.

China tried to keep out foreign missionaries before the Olympics. It kicked out more than 100 suspected missionaries last summer, according to a U.S. monitoring group, the China Aid Association. China’s intelligence services made lists of potentially troublesome evangelical Christians, and authorities tightened visa measures ahead of the games.

Taylor and other groups knew the risks.

Instead, the Christians came in on tourist visas and said they were involved in sports or cultural activities, which China allows.

Taylor’s group renovated a school in Yunnan province. Members then reached out to Chinese in one-on-one conversations.

One Christian group made headlines this week when Chinese authorities confiscated 315 Chinese-language Bibles found in their checked luggage when they arrived in the southwestern city of Kunming from Thailand.

Athletes stepped up to lead their own prayer groups or Bible studies after the Chinese said they would assign chaplains to the village’s religious services center instead of allowing teams to bring in their own, said Dan Britton, the fellowship’s senior vice president of ministries.

“It’s a very unique situation,” he said. “When you assign a chaplain, it’s almost like saying, ‘We’re bringing a team to China and assigning the coach.’ Well, the coach doesn’t know the players and only knows the sport. We feel the spiritual realm is the same way.”