I think this is evidence that most religious people know their religion is wrong.
Why assume the Old Testament is metaphorical, but assume the New Testament is literal? Neither claims to be allegorical. The common belief of the day was that the Jewish stories were literal and Jesus never told his disciples they weren't. He even said that the old laws are still to be followed.
The truth is, Christians don't want to practice the insane and violent laws of the Old Testament. So they just accept they're metaphorical, while not doing the same for the New Testament. And this isn't new, the Catholic Church was doing this during the Middle Ages.
Jesus was not a Christian. Jesus was a Jew of the apocalyptic branch of 1st century Judaism.
It was up to Christian theologians, the church fathers, the notable Christian philosophers of late antiquity to develop Christian theology, to interpret what the life of Jesus meant, and to reflect on what it means to be a Christian.
Christian theology does not start and stop with the OT and NT. Christian theology was developed over the course of centuries, through the writings and ecumenical councils of the early Christian fathers .
The life of the historical Jesus is confirmed by multiple independent sources. The OT is not comparable in terms of historical corroboration.
Mythology, allegory, and metaphor are important parts of being a human being. Even today. The Iliad, the Odyssey, Beowulf, Gilgamesh, the OT are some of the most important pieces of world literature ever created, containing much insight on the human condition, the nature of life, the ethical dimensions right-thinking humans should contemplate.
I for one do not begrudge anyone who looks to the OT either as an important work of literature and mythology, or as a source of spiritual truth through allegory.
I do disregard fundamentalist Protestants who view the OT as a literal description of historical events