This symbol only shows up a couple of times in the second part of the poem, but it's such an important and powerful one that we can't help but notice it. It's not terribly difficult to figure out what the intention is either. The snake represents evil or corruption, and the dove represents good or innocence. Coleridge isn't pulling any punches when it comes to communicating just what his intentions are here. What is most interesting about this symbol, though, is that Coleridge sets it up so the reader can easily understand its symbolic intention, but the character who needs to understand it the most completely misses the mark.
Lines 531-548: Bard Bracy is recounting his dream about seeing a white dove struggling the forest. It is important to note here that Bracy specifically tells Sir Leoline that the bird was called by his own daughter's name, meaning that the bird clearly represents the innocence of Christabel.
Lines 549-554: As the dream continues, Bracy realizes that there is a snake attacking the dove. Western audiences would probably immediately understand the snake as the bad guy here, considering the whole Satan-as-a-serpent thing. However, the fact that the snake is green throws a flag on the play. You see, beyond poetry, Coleridge is considered to be highly influential in bringing German philosophy into English-speaking culture. Fascinated with German philosophy, he learned the language in his mid-20s and was a voracious reader of German books and poetry. So what's all this got to do with a green snake? Well, shortly before Coleridge began writing "Christabel," a fellow named Johann Wolfgang von Goethe published a story called Das Märchen. The story is kind of complicated and definitely weird, but what's important is that the green snake is critical to the story. The green snake in Das Märchen represents a kind of bridge between the real world and the inner world of the other characters. The overall moral of the tale, so the speak, is that we cannot be happy until we are free to be who we truly are on the inside. Knowing Coleridge's interest in German literature and philosophy, and knowing that Goethe was a popular poet and essayist at the time, it isn't much of a stretch to connect the snake in Das Märchen and the snake here in "Christabel." The snake, then, may not represent evil, but that same ambiguous sexuality issue that Christabel may be wrestling with throughout the poem. Perhaps Christabel is truly attracted to women. Because she would be persecuted for that at the time, she is struggling with the feeling that this "evil" act with Geraldine is only her true self being exposed. The snake, then, is a symbol of evil to the speaker, but it's actually a symbol of her innermost secrets for Christabel.
Line 571: Here we see the effects of Geraldine's spell on Sir Leoline. While bard Bracy made it abundantly clear that the snake is Geraldine, the baron believes that the evil is actually the men who kidnapped her. Frankly, we don't even believe that there were any men at all, but the baron has the last word and sends Bracy out to find Geraldine's dad, anyway.
Lines 583-587: Christabel is able to see through Geraldine's spell and sees the woman for the evil snake she really is. Of course, Christabel is overcome with shame. Plus, she's the victim of an enchantment designed to keep her mouth shut. This combination of these things makes her confused and dizzy. Christabel keeps making snake noises too, which may be her way of communicating what she sees, despite the spell. Geraldine makes sure that Leoline isn't tipped off, though, by focusing her snakey voodoo on him.