Serra Redeemer's second ability checks the power of a creature only at the moment it enters the battlefield. If it enters with counters, those counters are included. If that creature's power is less than 2 when it enters the battlefield but becomes greater than 2 after the ability triggers, you will still put two counters on it.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.