During the Reformation in the 16th century, not only Slovakia, but most of Central Europe became Protestant. Martin Luther's reforms spread among Slovaks and Germans, while the acceptance of John Calvin's reforms was more usual among Hungarians. A translation of Luther's Catechism published in Bardejov, east Slovakia, in 1581, became the first Slovak printed book. In 1613, the Roman Catholic Archbishop Pazmany sent a dispatch to the Vatican from Trnava, west Slovakia, in which he said that "in Habsburg Hungary," i.e., mostly today's Slovakia, "hardly a tenth of the population are Catholics, the rest are of the Lutheran, or Calvinist persuasion."
During the counter-Reformation, most of the parishes became Catholic. But the Kingdom of Hungary remained somewhat more tolerant of non-Catholic Christians than the rest of the Habsburg lands, for a host of reasons. Essentially, the religion of the nobleman/landlord, but also his attitude, decided about the religion of the peasants on his lands. (That is why there was some influx of "political refugees" -- Protestants -- from among the Czechs during the early stages of the counter-Reformation.)
After the collapse of communism, a myth was sometimes repeated in the Western media (and even in the former Czech-Slovak Federation) about the Czechs being "more Protestant" in the past, as opposed to the traditionally Catholic Slovaks.
In reality, both the Slovaks and the Czechs were predominantly Catholic. Moreover, the Protestant minority was substantially more significant among the Slovaks than among the Czechs. After most restrictions on the Protestant Churches were lifted in all the Habsburg lands in 1781, dozens of Slovak Lutheran ministers from the Kingdom of Hungary embarked on missions to the Czech-speaking areas of the monarchy, aware that Protestantism had been practically eradicated there. Unlike most other Slavic nations, the Slovaks have been pluralistic in their religious life.
The official name of the Slovak Protestant Church is Evanjelicka cirkev augsburskeho vyznania na Slovensku, the "Evangelical Church of the Augsburg Confession in Slovakia." That is why in Slovak, the words evanjelik and luteran refer to the same creed. The appropriate English translation of either of these words when discussing Slovakia is Lutheran or Protestant, but not "evangelical," which has different connotations in (American) English.
Both the German and the Hungarian minorities in Slovakia have been predominantly Catholic like the Slovaks. The Germans in Slovakia had their own Lutheran Church before a large part of them were deported after WW II. Contrary to occasional conjectures, the Germans in Slovakia were almost all Roman Catholics -- the percentage of the Lutherans among them was just a fraction of what it was among the Slovaks. The Calvinist Church is formally called the Reformed Christian Church in Slovakia. Its members are mostly from the Hungarian minority, but it has a few Slovak members, too.
According to the 1930 census, 16.7% of the population of Slovakia were members of a Protestant Church, including 12% Slovak Lutherans, 0.04% German Lutherans and 4.4% Calvinists (another 71.6% were Roman Catholics). Communist statistics did not register religion, except in 1950 when 12.9% said they were members of the Slovak Lutheran Church and another 3.2% of the Calvinist Church.
The number of people who declare a religion has dropped since then. In the second post-communist census in 2001, 6.9% of the population said they were Lutherans (and 2.0% Calvinists; 68.9% Roman Catholics, 13% non-believers, 4.1% Greek Catholics -- i.e., the Byzantine Church in the U.S., 0.9% Eastern Orthodox).
For historical reasons, members of the largest Churches are not distributed evenly over Slovakia. Some are particularly dominant in certain areas. There are over 300 villages with a Lutheran majority. 25 districts (okres) have a higher-than-average percentage of Lutherans. The former counties in parentheses below are identified for historical, as well as current reference. The names, some in existence since the late Middle Ages, are still used in Slovakia to identify regions informally (similar to the use of, e.g., "New England" in the U.S.), but they were abolished as administrative units in 1920. The contemporary, much smaller districts are all named after the district capitals.
In Central Slovakia they include the districts of:
In West Slovakia the districts are:
In East Slovakia the districts are: