Goethe uses the metaphor of the "tree of life" as an allusion to the Biblical, Garden of Eden story, in which Satan, in the guise of a snake, persuades Adam and Eve to eat from the forbidden tree of knowledge. In Faust, Part II, Mephistopheles' actions mirror those of the snake, as he tempts Faust into an irreligious life of the senses and out of the favor of God.
Faust, Part II