Paul uses a literary device to connect the past with the present. But what exactly is an 'allegory,' and how does it work here?
The word 'allegory' (from Greek 'allos' meaning 'other' and 'agoreuo' meaning 'to speak') describes a way of speaking where one thing is said, but another is meant. It's like a continued metaphor where the literal story points to a deeper, spiritual truth. Paul isn't saying the story of Hagar and Sarah didn't happen historically. Instead, he's showing how that historical event in Abraham's life serves as a powerful 'type' or 'figure' that illustrates a spiritual reality about God's covenants.
Think of it this way: The Bible often uses real historical events to teach profound spiritual lessons. Paul is using the Genesis account of Hagar and Ishmael not as a made-up story, but as a divinely intended illustration of the contrast between two ways of relating to God: one based on law and slavery, and the other on grace and freedom.