Guardians' Pledge

• WWizards of the Coast2011-09-22
  Official Ruling
  Guardians' Pledge only gives a bonus to white creatures you control when it resolves. Creatures that enter later in the turn and nonwhite creatures that become white later in the turn will not get the bonus.
• WWizards of the Coast2011-09-22
  Official Ruling
  If a white creature that gets the bonus stops being white later in the turn, it will continue to get +2/+2 until end of turn.

Rulings via Scryfall