what does Mephistopheles mean when he exclaims " all places shall be hell that is not heaven "

Mephistopheles is warning Faustus about the extent of Hell. Faustus does not understand that Hell is everywhere for Mephistopheles because he has renounced God and Heaven permanently. Faust is not yet damned so this would not be the case for him yet. Mephistopheles, however, does convey what Faustus's life would be like should he be damned.