Heliod, the Radiant Dawn

If a spell you cast has {X} in its mana cost, you choose the value of X before calculating the spell’s total cost. For example, if a spell’s mana cost is {X}{R} and your opponents have drawn two cards this turn, you could choose 5 as the value of X and pay {3}{R} to cast the spell.

If there are additional costs to cast a spell, or if the cost to cast a spell is increased by an effect (such as the one created by Thalia, Guardian of Thraben’s ability), apply those increases before applying cost reductions.

The “as though they had flash” effect applies only to casting spells. It does not, for example, change when you may activate abilities that can be activated “only as a sorcery.”