By Christopher Landau
BBC religious affairs correspondent, Seoul
The Yoido Full Gospel Church was founded in 1958
There are few words to describe a visit to what is claimed to be the world's largest church congregation.
In just over 50 years, the Yoido Full Gospel Church has grown from five people meeting in its founding pastor's home in South Korea, to a membership of more than 750,000 people.
It means that the church, in Seoul, has more members than some entire denominations in Europe.
On a typical Sunday, more than 200,000 worshippers attend seven services in a building that feels more like a stadium than a traditional Christian structure.
Visiting the church felt like attending a major sporting event - without the crowd control challenges.
Inside, the atmosphere was more like an oversized concert hall, with a full orchestra and massed choir being strangely dwarfed by their surroundings.
Traditional hymns and contemporary songs were both sung, with words projected in Korean, English and Japanese onto (again, enormous) electronic display screens.
The services are recorded by TV cameras and broadcast on the internet
There were even 10 television cameras being directed from a control room, because services are broadcast to other churches and made available online.
So why is this church so successful?
According to its senior pastor, Young-hoon Lee, the church has grown because it preaches about how becoming a Christian will have a positive, tangible impact on every believer's life.
"Jesus will give us spiritual blessing, and prosperity, and physical health," he says.
The idea that Christianity automatically brings with it material gain is hotly contested by some, who condemn the idea as "the prosperity gospel".
I certainly could not help but notice that near the entrance of the church are two ATM-style machines, providing an easy way for members to deposit their weekly cash offering.
Pastor Lee says that so long as Christians give generously to charitable causes, there is no reason why they should not prosper and receive "blessing" themselves.
But I asked him whether he really meant that members of the church have improved physical health as a result of their faith?
"Many people still have problems, but many people overcome problems with faith," he told me.
A Christian-run clinic provides free healthcare for migrant workers
It is a faith that the thousands who attend here believe profoundly changes their lives.
And Pastor Lee has a dramatic prediction about the long-term impact of Christianity in South Korea.
"Our church is still growing, so sooner or later Christianity will be the major religion in Korea. All Christians are praying for that right now."
A few miles away, I witnessed a rather different approach to Christian life and service.
In a run-down building in the south of Seoul, hundreds of migrant workers were jostling for attention in a crowded clinic.
For many of them, the clinic - run by a Christian organisation - is the only place where they can obtain free healthcare.
On the top floor, those being treated are encouraged to attend a worship service - complete with upbeat songs and a rousing sermon.
There is a clear commitment from the staff at the centre to introduce those they are helping to the Christian faith.
Lee Sun Hee, the senior vice-president of the Love Village centre, tells me that the vast majority of patients accepts its Christian principles.
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"At our clinic, we spread the gospel. I'm not sure whether it's because of their desperation - but when we talk about Christianity, about 95% accept it.
"We don't refuse to give treatment because of their religion. We think God has given us a chance to provide help and serve people."
This sort of determined Christian commitment is not unusual in South Korea.
Another example is the huge number of Christian missionaries being sent to other countries - with some estimates putting the number at almost 20,000.
Some reports suggest that only the United States sends more Christian missionaries abroad than South Korea.
In 2007, a group of Korean Christians was kidnapped in Afghanistan - and two people were killed.
But stories such as these do not seem to be deterring South Korean Christians from signing up for missionary work.
I met Kim Pong Sik, who was preparing to leave South Korea. I asked him if he had concerns for his future security.
"I am very worried about it, actually. And every time I had any troubles in my mind, God has given me messages, that I should be bold and brave."
He also told me that it was "basic and natural" for him to be prepared to risk his life as a Christian missionary.
So what reasons are given for the rapid expansion of Christianity in South Korea?
Myoung-kyu Park, a sociology professor at Seoul National University, says Christianity is intrinsically associated with Western prosperity in the minds of many Koreans.
"Unlike Buddhism or Confucianism - traditional religious ways of thinking - Christianity could give Korean people very positive motivation," he says.
There is certainly little doubt that Christianity is having a significant impact on South Korean society - whether it is the red neon crosses that illuminate the Seoul skyline each evening, or the presence of church bands playing Christian music in the street.
South Korea is modernising rapidly, and embracing Christianity seems to be part of that process.
But at a time of such rapid social change, few can confidently predict the long-term place of Christian faith in the country's future.