When an opponent casts an enchantment spell, if Hidden Ancients is an enchantment, Hidden Ancients becomes a 5/5 Treefolk creature.
A noncreature permanent that turns into a creature can attack, and its {T} 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.
It changes into a creature even if the spell is countered.
When it turns into a creature, it is no longer an enchantment.