German Wikipedia says that

Four fifths of the inhabitants of Dresden do not belong to any religious community. Approx. 20 % of the population is predominantly Christian.

Original German text:

Etwa vier Fünftel der Einwohner gehören keiner religiösen Gemeinschaft an. Etwa 20 % der Stadtbevölkerung setzt sich mehrheitlich aus Angehörigen christlicher Konfessionen zusammen.

I expected the majority of Dresden inhabitants to be either Catholics or Protestants. According to this article, most of them (80 %) are not officially religious.

What events led to this? Why are 80 % of Dresden inhabitants atheists, agnostics, or "spiritual, but not religious"?

My first guess was the persecution of Christians by the Communists. If this hypothesis is true, we should see a comparable picture in other countries of the former Eastern bloc. I assume that Christianity was persecuted most harshly in Russia. In the Soviet Russia

• there were mass killings of Christian priests (contrary to Eastern Germany) and
• the Russians were exposed to atheist propaganda longer than Eastern Germans (1991 - 1917 = 74 years in Russia vs. 44 years in Eastern Germany).

If this logic is correct, there must be less Christians in Russia (percentage-wise) than in Dresden.

However, Wikipedia says that 41 % of Russians are Russian Orthodox, i. e. belong to a religious community that was persecuted the strongest during socialism. This means there are more than twice as many Christians in Russia (41 % Russian Orthodox, 47.1 %, if you count all Christian confessions) than in Dresden (20 %).

Therefore my original hypothesis is wrong (does not explain the low number of religious people in Dresden).

Update 1:

There are several other cities in Eastern Germany with similar parameters:

• Leipzig: 80 % of the population do not belong to any religious tradition (German: Heute gehören 80 % der Bevölkerung keiner Religion an.).
• Chemnitz: In year 2007 80 % of the population did not belong to any religion (Etwa 80 Prozent der Bevölkerung gehörten 2007 keiner Religion an.).
• Rostock: In 2016 10 % of the population were Protestants and 3.2 % Catholic. The vast majority of Rostock inhabitants is irreligious (Im Jahr 2016 waren 10 % der Rostocker Bevölkerung evangelisch und 3,2 % katholisch.[86] Die große Mehrheit der Rostocker ist hingegen konfessionslos.).
• Erfurt: In a representative household poll by the Erfurt's statistical authority in 2011 11 % of people said they were religious, and 81 % non-religous. In the cohort of people between 18 and 24 years old these numbers were 9 % and 88 %. In the cohort of people over 65 years, these were 14 % and 70 %. According to official data (Melderegister) of 2010, 13.7 % of population were Protestant, 6.5 % Roman-Catholic, 0.2 % Jewish, and 0.8 % belonged to a different religious tradition. (In der repräsentativen Haushaltebefragung der Erfurter Statistikstelle für 2011 bezeichneten sich 11 % der Befragten als religiös und 81 % als nicht religiös. Bei den 18- bis 24-Jährigen waren es 9 % und 88 % und bei den über 65-Jährigen 14 % und 70 %. [...] Nach Melderegister waren zum 31. Dezember 2010 etwa 13,7 % der Bevölkerung evangelisch, rund 6,5 % römisch-katholisch, nur 0,2 % jüdisch und 0,8 % gehörten einer anderen registrierten Religionsgemeinschaft [...] an.).