In revealing his mysterious name, YHWH (“I AM HE WHO IS”, “I AM WHO AM” or “I AM WHO I AM”), God says who he is and by what name he is to be called. This divine name is mysterious just as God is mystery. It is at once a name revealed and something like the refusal of a name, and hence it better expresses God as what he is – infinitely above everything that we can understand or say: he is the “hidden God”, his name is ineffable, and he is the God who makes himself close to men.
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.
Faced with God’s fascinating and mysterious presence, man discovers his own insignificance. Before the burning bush, Moses takes off his sandals and veils his face in the presence of God’s holiness. Before the glory of the thrice-holy God, Isaiah cries out: “Woe is me! I am lost; for I am a man of unclean lips.” Before the divine signs wrought by Jesus, Peter exclaims: “Depart from me, for I am a sinful man, O Lord.” But because God is holy, he can forgive the man who realizes that he is a sinner before him: “I will not execute my fierce anger. . . for I am God and not man, the Holy One in your midst.” The apostle John says likewise: “We shall. . . reassure our hearts before him whenever our hearts condemn us; for God is greater than our hearts, and he knows everything.”
Out of respect for the holiness of God, the people of Israel do not pronounce his name. In the reading of Sacred Scripture, the revealed name (YHWH) is replaced by the divine title “LORD” (in Hebrew Adonai, in Greek Kyrios). It is under this title that the divinity of Jesus will be acclaimed: “Jesus is LORD.”
From the Catechism of the Catholic Church 206-209