You can channel divine energy to counter spells without readying an action in advance.

<php>echo “<b>Prerequisites:</b>”;</php> Channel Energy, divine caster level 3rd
<php>echo “<b>Benefit:</b>”;</php> You can spend one of your uses of channel energy as an immediate action to counter a spell or spell-like effect. For example, if an evil cleric targets Jozan with a hold person spell, as an immediate action, Jozan can spend a turn undead attempt to counter the spell if he had prepared a hold person or dispel magic spell. You must have the relevant spell prepared as normal (or dispel magic), and you must make an Arcana or Religion check to identify the target’s spell if applicable. (See PH 170 for details on counterspells.)
<php>echo “<b>Normal:</b>”;</php> You must typically use a readied action to counter an enemy’s spell.