Songfacts®:

• On January 10, 49 BC, Roman Governor Julius Caesar led his army across the Rubicon River in direct opposition to the orders of the Roman Senate, triggering the Roman Civil War. Caesar knew this would happen. Upon completing the crossing, he reportedly uttered, "And so the die is cast." In the ensuing conflict, Caesar became the sole dictator of Rome.
  
  Caesar's crossing of the Rubicon was so historically significant that, to this day, there's a common saying for it. To "cross the Rubicon" means to cross a point of no return. It means a person has definitively committed him or herself to a course of action, and there is no turning back.
  
  Dylan is using the notion of "crossing the Rubicon" in the latter general sense rather than the former historical one. The whole song makes this pretty clear, but there's also a detail in the very first line that gives it away.
  
  I crossed the Rubicon on the 14th day
  Of the most dangerous month of the year
  
  Caesar made his crossing on January 10, not 14. Which month is "most dangerous month of the year," we don't know.
• Dylan's intended meaning (if indeed there is an intended linear meaning) is hard to see in this one, but there are some clues. We also can make assumptions based on the context of the Rough And Rowdy Ways album, which is heavy on Biblical imagery and Christian ideas, as heard in "False Prophet," "Goodbye Jimmy Reed," and "Murder Most Foul."
  
  One verse, in particular, gets the Christian viewpoint across:
  
  I feel the Holy Spirit inside
  See the light that freedom gives
  I believe it's in the reach of
  Every man who lives
  Keep as far away as possible
  It's darkest 'fore the dawn (Oh Lord)
  I turned the key, I broke it off
  And I crossed the Rubicon
  
  The "holy spirit" is frequently referenced in Christianity. It's basically the "spirit of God" and can inhabit the faithful or convert the unfaithful. It's what empowers Baptists to handle deadly snakes and evangelicals to sing their praises.
  
  Freedom ("see the light that freedom gives") is also a concept Dylan has discussed in relation to "finding Jesus." It may seem strange to non-Christians (who tend to view the faith as an authoritarian one), but for many, Jesus promises freedom. It's not freedom in a political sense, but freedom from sin, which can be viewed in all sorts of ways (from freedom from "the fire and brimstone of Hell" to the same internal "freedom from self" sought by many Eastern religions).
  
  It's tempting to say Dylan is referring to his own Christian conversion experience, but we really don't have any evidence for that beyond the fact that he did convert (some time back in the '70s) and that his song seems to be discussing "crossing the Rubicon" into Christian faith.
  
  For a taste of Dylan's most overtly Christian music, check out songs like "Pressing On" and "Gotta Serve Somebody."
• Three miles north of purgatory
  
  In the Roman Catholic faith, Purgatory is the place where sinners' souls go before going up to Heaven. The word "purgatory" has become common slang for a lost place (either physical, mental, or spiritual) with no definitive direction.
• Mona, baby, are you still in my mind?
  
  We haven't heard from Mona since the 1966 song "Stuck Inside of Mobile with the Memphis Blues Again," where:
  
  Mona tried to tell me
  To stay away from the train line
  She said that all the railroad men
  Just drink up your blood like wine
  
  Of course, Dylan can be talking about a different Mona, but it seems unlikely seeing as "Mona" is not a very common name these days and "Stuck Inside of Mobile with the Memphis Blues Again" is one of Dylan's most well-known songs. At the very least, Dylan had to know fans would automatically make the connection back to the older song.