The Tower of Babel in Genesis 11 was waterproofed to withstand a future flood. St. Paul mocks the Roman Empire in Acts 26. And King Herod's appearance in Matthew 2 explains the context of Jesus's arrival on earth. But these and many other central facts about the Bible's narrative escape the modern Bible reader.

The problem is that background information that was once too obvious to write down has now been largely forgotten.

Just as modern publications don't remind readers that France is in Europe, or that Hitler was evil, or that Democrats and Republicans disagree, some facts were so widely known 2,000 years ago that there was no reason to include them in the Bible. But readers today lack this once-common knowledge, so they are often unable to appreciate the original depth of the Bible. Fortunately, other ancient sources fill in the blanks, restoring the Bible's original impact. Here are three examples.

According to Genesis 11:3, the Tower of Babel was crafted with brick for stone and bitumen for mortar. Modern readers pay little attention to these engineering specifications, so they don't appreciate the importance of the bitumen.

A 1st-century AD historian named Josephus helps out. He explains that the bitumen was to make sure that water didn't enter the tower. Bitumen, it turns out, was a common waterproofing agent, used here as well as for Noah's ark and for the basket that carried Moses safely down the Nile. The point of the Tower of Babel was to defend against a future flood, and Babel marks the end of the Flood narrative. But modern readers who are not experts in ancient materials sciences miss the role of the Tower and its connection to the Flood.

A second example comes from St. Paul's imprisonment. After being arrested in connection with his disputes with some Jews, the Christian leader defends himself before a series of Roman rulers. Modern readers generally ignore the names Felix, Festus, and Agrippa and focus instead on Paul's words. But the names are the key to understanding what Paul says.

Acts 25:13 refers to King Agrippa and Bernice who travel together. The obvious assumption is that Bernice is Agrippa's queen, and that she has nothing to do with Paul's observation in Acts 26 that Agrippa is especially familiar with all the customs of the Jews.

But we learn from Josephus that Bernice is actually Agrippa's sister, and that the king is having an incestuous affair, in violation of the most basic of Jewish (and Christian) customs. Paul publicly praises the Roman for his knowledge of all things Jewish while inwardly ridiculing the man for his most heathenly, non-Jewish (and non-Christian) behavior. The trial is a farce. But most modern readers don't know it.

Thirdly, few biblical passages are more familiar than Matthew 2:1: In the time of King Herod , wise men, or magi, came to Jerusalem after Jesus was born. Modern readers focus on Jesus and on the wise men, and disregard Herod. But Herod is central here.

Herod was widely seen in his day as an especially vicious Roman tyrant. For ancient readers, the brief reference to King Herod would have evoked images of a heathen monarch smothering God's holy city of Jerusalem, much the way Stalinist Russia sets a dismal tone today.

Matthew's point in chapter 2 is not just that Jesus has been born. It's that a savior has arrived just at the time when the denizens of Jerusalem most need him. In this context, the arrival of the Messiah takes on heightened importance.

In these places and many more, the full impact of the Bible comes through only in light of the background to the text. But readers have to search elsewhere for that background.

Dr. Hoffman is author most recently of The Bible's Cutting Room Floor , which explores the relationship between extra-biblical material and the Bible itself. He can be reached through his website at www.lashon.net .