The earliest-identified church is actually a home in Dura-Europos, or modern-day Syria. Known as the Dura-Europos church, this structure was a family home discovered during an archeological dig in the early 1900s.
Before Christianity was legalized in 313 AD, many Christians gathered secretly in private homes like the Dura-Europos house church for Mass. This structure was converted for worship between 233 and 256.
The Dura-Europos church is similar to churches as we know them today, containing religious paintings of David and Goliath, Adam and Eve, and Christ as the Good Shepherd. The baptistry room contains frescoes that were painted around 235 and are likely some of the most ancient Christian paintings.
To learn more about early Church history, check out 101 Surprising Facts About Church History by Fr. David Vincent Meconi, S.J., sold here.