Threshold — As long as seven or more cards are in your graveyard, Cephalid Sage has "When Cephalid Sage enters the battlefield, draw three cards, then discard two cards."
6/8/2016 You draw three cards and discard two cards all while Cephalid Sage’s ability is resolving. Nothing can happen between the two, and no player may choose to take actions.
6/8/2016 If you don’t have enough cards in your graveyard at the moment Cephalid Sage enters the battlefield, its threshold ability won’t trigger. If the number of cards in your graveyard changes and Cephalid Sage loses the ability once it has triggered, the ability will still resolve.