Opal Acrolith

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 their most recent turn. It doesn't matter how long the permanent has been a creature.

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.

When it turns into an enchantment, it is no longer a creature.

Changes into a creature even if the spell is countered.

When it turns into a creature, it is no longer an enchantment.

It triggers when the spell is cast, which means it becomes a creature before that spell resolves.