The Temple has a kind of ability called a characteristic-defining ability. It has a printed ability that defines its own power and toughness, does not directly affect any other cards with its ability, and does not set its power only if a certain condition is met. A characteristic-defining ability works in every zone, not just the battlefield. So no matter where the Temple is, its power and toughness are equal to the number of creatures you control, even while it’s on the stack. So in this case, while the Temple is on the stack, it is a 5/5 because you control 5 creatures (though it would be a 6/6 once it entered the battlefield, it won’t count itself if it’s not on the battlefield yet). When the Backlash looks at the spell’s power, it sees its power was 5, so you’ll still take 5 damage.
Today’s Rules Tip written by Nate Long