Blood

When Faustus makes his pact with the devil, he signs it with his own blood. But as he goes to write, his blood congeals. As disgusting as that is, it's also a key moment in the play.

Faustus wonders, "What might the staying of my blood portend? / Is it unwilling I should write this bill?" (2.1.62-63). He thinks the fact that his blood is congealing and clotting is some sort of sign. He thinks it means something.

At this point, you might even say that Faustus thinks his sticky blood is a symbol. Of what? Of the part of himself that's unwilling to give it all up for the devil.

And let's not forget that Lucifer insists on having the document written in blood. To him, this blood represents Faustus's person. If he has the blood, he has Faustus. So it's a symbol to Lucifer, too. Without the blood, there's nothing physical to represent Faustus's soul.

Then, at the end of the play, as he waits for death, Faustus cries out, "One drop of blood will save me. O, my Christ!" So now we've got someone else's blood in the mix—the blood of Christ to be precise. That salvation he's talking about? It's the one mankind gained when Christ traded his human body and soul for those of the sinful people in hell, according to Christianity. Here, again, blood is a symbol of the whole person—Christ.