Left stamp - Cathedral of Gniezno
The stamp was issued on 1 March 2002. It is also showing the coffin with relics of St. Adalberg.
The Cathedral of Gniezno, according to wikipedia, served as the coronation place for several Polish monarchs and the seat of Polish church officials continuously for nearly 1000 years.
Right stamp - Fortified Church of Biertan
The stamp was issued on 11 September 2011. The Fortified Church of Biertan was declared as a UNESCO World Heritage Site in 1993 under the category: villages with fortified churches in Transylvania.
For more places of worship stamps, visit See it on a Postcard.