A selected quote from the Catechism of the Catholic Church in honor of the Year of Faith (October 11, 2012 - November 24, 2013)
201 To Israel, his chosen, God revealed himself as the only One: "Hear, O Israel: The LORD our God is one LORD; and you shall love the LORD your God with all your heart, and with all your soul, and with all your might." Through the prophets, God calls Israel and all nations to turn to him, the one and only God: "Turn to me and be saved, all the ends of the earth! For I am God, and there is no other.. . To me every knee shall bow, every tongue shall swear. 'Only in the LORD, it shall be said of me, are righteousness and strength.'"
202 Jesus himself affirms that God is "the one Lord" whom you must love "with all your heart, and with all your soul, and with all your mind, and with all your strength". At the same time Jesus gives us to understand that he himself is "the Lord". To confess that Jesus is Lord is distinctive of Christian faith. This is not contrary to belief in the One God. Nor does believing in the Holy Spirit as "Lord and giver of life" introduce any division into the One God:
We firmly believe and confess without reservation that there is only one true God, eternal infinite (immensus) and unchangeable, incomprehensible, almighty and ineffable, the Father and the Son and the Holy Spirit; three persons indeed, but one essence, substance or nature entirely simple.203 God revealed himself to his people Israel by making his name known to them. A name expresses a person's essence and identity and the meaning of this person's life. God has a name; he is not an anonymous force. To disclose one's name is to make oneself known to others; in a way it is to hand oneself over by becoming accessible, capable of being known more intimately and addressed personally.
206 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.
207 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").12 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.
208 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."
214 God, "HE WHO IS", revealed himself to Israel as the one "abounding in steadfast love and faithfulness". These two terms express summarily the riches of the divine name. In all his works God displays, not only his kindness, goodness, grace and steadfast love, but also his trustworthiness, constancy, faithfulness and truth. "I give thanks to your name for your steadfast love and your faithfulness." He is the Truth, for "God is light and in him there is no darkness"; "God is love", as the apostle John teaches.
215 "The sum of your word is truth; and every one of your righteous ordinances endures forever." "And now, O LORD God, you are God, and your words are true"; this is why God's promises always come true. God is Truth itself, whose words cannot deceive. This is why one can abandon oneself in full trust to the truth and faithfulness of his word in all things. The beginning of sin and of man's fall was due to a lie of the tempter who induced doubt of God's word, kindness and faithfulness.
218 In the course of its history, Israel was able to discover that God had only one reason to reveal himself to them, a single motive for choosing them from among all peoples as his special possession: his sheer gratuitous love.38 And thanks to the prophets Israel understood that it was again out of love that God never stopped saving them and pardoning their unfaithfulness and sins.