Dura-Europos Church is widely considered to be the oldest in existence.
More specifically, Dura-Europos is known as the world's oldest Christian house church, as it was originally constructed as a home before converting into a church.
A house church is defined as a place where people gather for worship together in a private home.
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Since Dura-Europos was not originally constructed as a church, there are historians that don't believe it should hold that status that it widely does.
That said, there are still many sources that credit Dura-Europos with the "world's oldest" title. The church is named the oldest church building by Guinness World Records.
The church was built in the city of Dura-Europos, near the village of Salihiyah in modern-day Syria. The church was built somewhere between 233 and 256 AD, according to Christianity.com
In the 1920s and 1930s, a group of American and French archaeologists, led by Yale University and the French Academy of Inscriptions and Letters, excavated the area where thousands of artifacts, including frescoes, were unearthed. Fresco painting is a "method of painting water-based pigments on freshly applied plaster, usually on wall surfaces," according to Britannica.
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Excavating the city led to the discovery of an unearthed synagogue, a Christian building and Mithraeum, plus over 12,000 artifacts that are preserved at Yale University today, according to the Ivy League's website.
The paintings found during excavation are among the artifacts that now live at Yale University and are considered some of the oldest Christian paintings ever discovered.
One of these is a piece that was found in the baptistery, which was discovered in three fragments.
Today, the church is left in ruins in an area controlled by ISIS, according to Christianity.com.
As for the oldest church in the United States, many sources consider it to be San Miguel Chapel in Santa Fe, New Mexico.
The Catholic Church dates back to 1610. The chapel has been reconstructed many times since it's creation and is still in operation today.