Magic has a ton of spells and abilities that give creatures abilities, affect their power and toughness and change other characteristics of those creatures. It is important to note that continuous effects that are generated when a spell or ability resolves will only affect those creatures that are on the battlefield at that time. In other words there is a difference between Guardians’ Pledge and Honor of the Pure.

Honor of the Pure is an enchantment with a static ability that generates a continuous effect. This effect is applied to white creatures you control for as long as Honor the Pure is in play. If a white creature enters the battlefield under your control after Honor the Pure has been there for a while, it will still get the +1/+1 bonus.

While Guardians’ Pledge also generates a continuous effect, how it works is very different. It is a spell that generates a continuos effect when it resolves. This effect also has a duration that is determined when the effect is created, it will end when the current turn does. Finally, while it does affect all white creatures you control the +2/+2 bonus will not be applied to a creature that enters play after this effect is generated.