In God “there is no variation or shadow due to change.” God is “HE WHO IS”, from everlasting to everlasting, and as such remains ever faithful to himself and to his promises.
The revelation of the ineffable name “I AM WHO AM” contains then the truth that God alone IS. the Greek Septuagint translation of the Hebrew Scriptures, and following it the Church’s Tradition, understood the divine name in this sense: God is the fullness of Being and of every perfection, without origin and without end. All creatures receive all that they are and have from him; but he alone is his very being, and he is of himself everything that he is.
From the Catechism of the Catholic Church 212-213