Mary and Martha are the sisters of Lazarus, whom Jesus raised from the dead (John 11:1-44). The lesson in that passage, as in the story of the Israelites' escape from Egypt, is about faith in the power of Jesus, or God, to perform miracles. The point seems to be that no matter how insurmountable your problem seems to be, God can handle it.
I agree with Murray that, although the rock could be the mountain Moses climbed, it's probably a reference to the parting of the Red Sea. "Linin' Track" includes that reference to Moses ("If I could, I surely would...") as well as "Moses stood on the Red Sea shore/And smote that water with a two-by-four." Obviously a popular image.