It's important that we know where and how we came to be. For example we wouldn't really understand why you're a certain religion if we didn't know who settled the land and what happened. Not only that, but also a lot of things happened back then, a civil war, WWI and WWII, the Cold war, war of 1812, that made things what it is present day. Also history makes sure that we can honor people for freedom, such as Abe Lincoln who freed slaves. The point I'm making here is that history is a way to remember important events that have effected many things and many places and without keeping track of history (for example if we didn't keep track of the Constitution) we wouldn't have the somewhat peaceful society of today.
It's mainly important so that we don't repeat our past mistakes. For example,the JFK assassination, we let one president get killed but we learned to not let that happen anymore.
Before Constantine. The first recorded official persecution of Christians on behalf of the Roman Empire was in AD 64, when, as reported by the Roman historian Tacitus, Emperor Nero attempted to blame Christians for the Great Fire of Rome.
