Text (M10+errata): 1/1. ; Whenever another creature enters the battlefield, you gain 1 life. [Oracle 2009/07/08]
The ability will not trigger on itself coming into play, but it will trigger on any other creature that is put into play at the same time Soul Warden is, or while it is in play. [D'Angelo 1999/08/01]
Does not trigger on a card in play being changed into a creature. [D'Angelo 1999/02/23]
Note - This card was of creature type "Cleric" and not "Human Cleric" until Ninth Edition and Oracle 2005/08/01.