"I Am who I Am"
By revealing his name God at the same time reveals his faithfulness which is from everlasting to everlasting, valid for the past ("I am the God of your father"), as for the future ("I will be with you"). God, who reveals his name as "I AM", reveals himself as the God who is always there, present to his people in order to save them.
Source: Catechism of the Catholic Church (Vatican.va)