Whenever an opponent casts a creature spell, if Opal Acrolith is an enchantment, Opal Acrolith becomes a 2/4 Soldier creature.
: Opal Acrolith becomes an enchantment.
10/4/2004 When it turns into an enchantment, it is no longer a creature.
10/4/2004 Changes into a creature even if the spell is countered.
10/4/2004 When it turns into a creature, it is no longer an enchantment.
10/4/2004 It triggers when the spell is cast, which means it becomes a creature before that spell resolves.
4/1/2008 A “creature spell” is any spell with the type Creature, even if it has other types such as Artifact or Enchantment. Older cards of type Summon are also Creature spells.
8/1/2008 A noncreature permanent that turns into a creature can attack, and its abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of his or her most recent turn. It doesn’t matter how long the permanent has been a creature.