Silas's rebirth doesn't take place on Christmas.
See, that's actually important. The main story of Silas's redemption—betrayal, the arrival of a child, and then the reintegration into the community—is pretty much exactly the story of Christian fall and salvation. The soul is cast out of God's company because of sin and then is brought back through Jesus, who is born as a human on Christmas. In that sense, Silas's story is a clear allegory of salvation.
But it doesn't take place on Christmas. It takes place just after Christmas, or, as the narrator says, "about the Christmas of that fifteenth year" after Silas comes to Raveloe (1.2.11). That's a clue telling us to be careful about dismissing Silas Marner as pure allegory. Allegory is only part of the story. Godfrey and Nancy inhabit a completely different type of novel—one where symbols can't be broken down so neatly, and one where the right moral decision isn't so easy to make.
In fact, you might say that Eliot is thinking about the very nature of allegory. How satisfying are allegorical stories? Is it possible to write a complex, sophisticated novel based on an allegorical story? What happens when you try to make an allegory story seem real? (Hint: you get something a lot like Godfrey's narrative arc.)