Warsaw Confederation was important event in history of Poland, which is considered as beginning of religious freedom in Poland (January the 28th, 1573).
The religious tolerance in Poland had much longer tradition and was de facto policy during the reign of recently deceased king Sigismund II of Poland. However, the articles signed by confederacy gave official sanction for what was earlier simply custom. In that sense, it may be considered either the peak of Polish tolerance, or as beginning of it.
After the childless death of last king from dynasty of Jagiellons, nobility ammassed in Warsaw and signed document, in which representants of each major religion promised each other support and tolerance. The major difference between this act and other similar was that act was not imposed by government, or as a result of war, but rather as effect of good will of the society.
Most important person tied to preparation of the articles were Mikolaj Sienicki, leader of execution movement, Jan Firlej and Jan Zborowski
Articles of Warsaw Confederacy were later included into Henrykian articles becoming part of those first Polish constitution.
There is debate whether religious freedom was intended only for nobility, or for peasants and others as well; generally most of historians tend to agree with the latter.