The answer is Syria. Dura Europos to be exact.

From Sacred Destinations:

Dura-Europos also boasts the oldest known Christian church. It was dismantled and re-constructed at Yale University in the early 1930s, so there isn’t much to see at Dura-Europos but basic foundations.

The church occupied a typical Roman upper-class house centered around a columned courtyard with an open room (atrium). In the center of the courtyard was a pool (impluvium). At the opposite end from the entrance was a raised area (tablinum) containing a table and used by the family as a reception area and for ceremonial functions.

Scholars speculate that the congregation gathered around the pool, which was used for baptism. In the tablinum sat the bishop, who celebrated the Eucharist (communion) at the table. This arrangement provides a basis for the liturgical arrangement of later basilica churches.

The murals of the Dura Europos chuch were painted between 232 and 256 AD and are among the earliest examples of Christian art that survives today. The mural of the Healing of the Paralytic contains the earliest image of Jesus found anywhere.

In 1933, an important fragmentary text was unearthed at Dura Europos that contained a previously unknown Greek harmony of the gospels, dated to the late 2nd century. This has been important for early Christian studies, particular those of Tatian’s Diatessaron, a more well-known gospel harmony.