Dozens of journalists were invited into the prison on Sunday to view two well-preserved tile mosaics, which include detailed inscriptions in Greek and which the authority said served as the floor of the church.
"It is for sure the earliest church in Israel that we know of," said Yotam Tepper, the archaeologist in charge of the dig, which began seven months ago.
The announcement was met with deep skepticism from some scholars of early Christianity.
The traditional view is that Christian churches did not begin to appear in the region until the fourth century A.D., the result of Emperor Constantine's edict in A.D. 313 that Christians could worship freely in the Roman Empire.
Before that, Christians were often persecuted. They worshiped clandestinely and were not able to build public houses of worship, these scholars say.
"For people who study this, it would be very hard to accept that there is a Christian church here that dates to the third century," said Joe Zias, an anthropologist and a former curator with the Antiquities Authority. Mr. Zias, who has not seen the site, added, "My gut feeling is that we are looking at a Roman building that may have been converted to a church at a later date."
Pottery shards from cooking pots and wine jugs resting on the mosaic have been dated to the late third century A.D., suggesting the mosaic - and presumably the church - was already in place at that time, he said. The style of the Greek lettering in the three inscriptions point to the same period, he said, and the structure does not follow the traditional building pattern for churches that emerged in the fourth century.
The floor is about 30 feet by 15 feet and has two mosaics, consisting of small black and white tiles in geometric patterns. Two fish, a symbol widely used in early Christianity, adorn one.
In the center of the floor is a base that may have supported a structure used in worship services, Mr. Tepper said. Nearby, one inscription reads, "The God-loving Aketous has offered this table to the God Jesus Christ, as a memorial," according to a preliminary translation by the Antiquities Authority.
Another inscription says a Roman military officer, Gaianus, "having sought honor, from his own money, has made the mosaic."
But Mr. Zias said it struck him as strange that a Roman military officer would take credit at a time when the Roman authorities prohibited practicing Christianity. "If I were a Roman soldier in the third century, I certainly wouldn't want my name on it," he said. "This would not have been a good career move. In fact, it sounds like the kiss of death."
If the Megiddo site does date to the third century, "then I would ask why it was not reported or discussed by early church historians," said Yiska Harani, a historian with expertise on Christian pilgrimage to the Holy Land. "How did they overlook a successful place of early worship?"
Mr. Tepper said no decision had been made on the fate of the site. He said he hoped it could become a small museum, while acknowledging the problems of doing so inside a prison.
"We just don't know what will happen at this point," he said.
The whole thing (including another photo of the mosaic) ....