There were and are churches there (with the pause for being bombed to rubble by USA) but they weren't at any point even close to being "christian nation", let alone "one of the most christian nations". There were below 10% of christians in Korea around 1950, and the % in DPRK is even less now, it was below 2% at 2005. Even occupied part of Korea, where there is some 30% christians now is nowhere near to be called "christian nation".

EDIT: there are 5 churches in Pyongyang officially, number similar to large village or small town in let's say Poland.

Kim Il-Sung himself grew up Presbyterian and dedicated a rebuilt church to his mother after the war. Christianity in Japanese-occupied Korea was actually organized against the empire and was known for sprouting anti-authoritarian movements.

Christian news sources claim there were 3000 churches in North Korea before 1945-1953 (the end dates conveniently change depending on who you ask, but they overlap with wars which tend to be accompanied by human migrations). Pyongyang was supposedly called the "Jerusalem of the East" by contemporaries. All sources agree that after the Korean War there were no churches in the North.

Kim Il-Sung ordered some to be rebuilt, but churches with overt American influence were not allowed to take root. His personal statement on the fall of Christianity in the North was that the people couldn't bear seeing the atrocities committed by fellow Christians. I think it's something in the middle where American missionaries had an incentive to route their flocks to the south to support their fellow Americans, and the believers themselves were simply escaping the egregious American bombing campaigns. The rest is embellishment.

Despite people saying you can't be Christian in North Korea, there are pictures of the churches online. You can go to them. There's a seminary even. So it calls into question the idea of state repression in its entirety because you can refute it with the simple facts in front of your face.