Source: https://secondexodus.com/the-catechism/god-of-israel/the-holy-trinity/
Timestamp: 2019-04-19 10:34:48+00:00

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“Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit” Mt 28:19.
BeShem ha’Av, ha’Ben, veRuakh haKodesh. In the name of the Father, the Son, and the Holy Spirit.
The Trinity Shield is a good visual to accompany God’s own simple explanation for his people Israel in Moses’ time.
The Shma, Judaism’s signature prayer, presents God as triune, one family of three divine persons! God sets apart its extraordinary importance with the opening words “Hear O Israel!” The Shma says: “Adonai [my Lords] Eloheinu [our Gods] Adonai [my Lords] ekhad [one]” Deut 6:4. These words are plurals. God is saying one in three. If he had meant one not three, he would have said yakhid, the Hebrew word for an absolute unity. But he said ekhad, the Hebrew word for a compound unity.
“As the Father has sent me, even so I send you” Jn 20:21. We generally use God‘s words to teach his lessons when we can. However, Maimonides, in his Thirteen Principles of Faith, implicitly recognized that ekhad could be used to support God as a Holy Trinity. God’s Shma said ekhad [compound unity], but Maimonides substituted yakhid, [absolute unity]. If a sage of Maimonides stature believed that ekhad supports the Holy Trinity, so should every Jew who reveres his teaching.
The Father is made of none, neither created nor begotten. The Son is of the Father alone; not made nor created, but begotten. The Holy Spirit is of the Father and of the Son; neither made, nor created, nor begotten, but proceeding. Yet each is fully God. Not part of God. The Father contains within him the Son and the Holy Spirit. The Son contains within him the Father and the Holy Spirit. The Holy Spirit contains within him the Father and the Son.
If he inquires still further, we may invite him to read all of the Athanasian Creed (below), and the dogma (further below).
Rabbi Yeshua told Shimeon, “Put out into the deep and let down your nets for a catch” Lk 5:4. To penetrate more deeply into the Holy Trinity, Second Exodus suggests the Catechism, § 232-267.
To savor the flavor of Pope Leo XIII’s and St. John Paul II’s observations on the Holy Trinity, Second Exodus suggests: Pope Leo XIII’s Divinum Illud Munus, 1897, on the Holy Spirit, addresses the Holy Trinity most especially at § 3, but also at 2, 4, 9. St. John Paul II’s Dominum et Vivificantem, 1986, on the Holy Spirit, addresses the Holy Trinity at § 9, 11, 12, 17, 39, 45, 48, 58, 66, 67. John Paul’s Ecclesia de Eucharistia, 2003, addresses the Holy Trinity at § 8, 16, 30, 43, 50, 60.
Finally, to watch the development of doctrine on the Dogma of the Holy Trinity, Second Exodus recommends Catholic Answers’ tract on The Trinity.
A mystery is a divinely revealed truth whose very possibility before it is revealed cannot be rationally conceived.
After a mystery is revealed its inner essence is incomprehensible. It cannot be fully understood by the finite mind because it is a manifestation of God, who is infinite and therefore beyond the complete grasp of a created intellect.
A mystery is incomprehensible, but it is intelligible. One of the primary duties of a believer is, through prayer, study, and experience, to grow in faith, to develop an understanding of what God has revealed.
The most central mystery in the hierarchy of truths! God loves us so much that he has revealed to us more than we can understand. But, because we love God so much, we try valiantly to understand as best we can. If we get the Dogma of the Holy Trinity right, it will lead us to the rest of “the faith which was once for all delivered to the saints” Jude 3. If we misunderstand, we will encounter confusion further along the road.
Some Catholics, pressed by tough questions, say, “It’s a mystery, beyond our understanding.” However, in Catholic teaching, a mystery is not unknowable but incomprehensible, a secret truth or plan hidden from the common knowledge of men.
§ 237 God reveals mysteries to us. Rabbi Yeshua speaks of mysteries of the kingdom of God Mt 13:11; Mk 4:11; Lk 8:10 which he disclosed to his shlikhim but not to the crowds who heard them only in parables, “So that they may indeed see but not perceive, and may indeed hear but not understand” Mk 4:12. God always has reasons for revealing to some but not to others, though we may not understand them at the time.
Jews had prayed the Shma since Moses’ time, but our Father emphasized one God, to break their adherence to Egypt’s many false gods. Jews say the Shma is the basis for their belief in one God as opposed to one triune God. Later, fulfilling Isaiah’s Prophecy, some Jews did not follow Rabbi Yeshua.
Often we are tempted to explain the Holy Trinity by an analogy. Rabbi Yeshua’s analogies Mt 11:16; 13:24; 18:23; 22:2; 25:1 work because he presents them as parables. However, God who is not created is unlike anything in his creation, so human analogies can’t work.
Our one God is three distinct persons § 254. Modalism is the false belief, proposed by Sabellius, that God is one person who has revealed himself in three modes of existence. But St. Irenaeus taught, “He is a simple, uncompounded Being, without diverse members” (Against Heresies, 2:13:3, AD 189). Pope Callistus I excommunicated Sabellius for his heresy in AD 220.
Some people still offer modalist explanations of the Holy Trinity. A popular but wrong idea is that the Holy Trinity is like a man who is at once a son, a father, and an uncle, but that man is one man with three titles. Still another is that the Holy Trinity is like water, which can exist as ice, liquid, or steam. But the three divine Persons of the Holy Trinity coexist, the different forms of water do not coexist. Water is not ice and liquid at the same time (when ice melts each molecule is either ice or water at any given moment).
Sometimes, when this is pointed out, people overcompensate by emphasizing God’s oneness. However, that still forces every analogy to explain God’s triunity by proposing parts. For instance, some will say that the Holy Trinity is like a three-leaf clover. If we look at its three leaves it is three, but if we look at the plant alone it is one. The explanation remains tangled in the modalist heresy.
The Catechism tells us about God.
§ 198 Our profession of faith begins with God, for God is the First and the Last, The beginning and the end of everything. the Credo begins with God the Father, for the Father is the first divine person of the Most Holy Trinity; our Creed begins with the creation of heaven and earth, for creation is the beginning and the foundation of all God’s works.
§ 199 “I believe in God”: this first affirmation of the Apostles’ Creed is also the most fundamental. the whole Creed speaks of God, and when it also speaks of man and of the world it does so in relation to God. the other articles of the Creed all depend on the first, just as the remaining Commandments make the first explicit. the other articles help us to know God better as he revealed himself progressively to men. The faithful first profess their belief in God.
§ 200 These are the words with which the Niceno-Constantinopolitan Creed begins. The confession of God’s oneness, which has its roots in the divine revelation of the Old Covenant, is inseparable from the profession of God’s existence and is equally fundamental. God is unique; there is only one God: The Christian faith confesses that God is one in nature, substance and essence.
§ 205 God calls Moses from the midst of a bush that bums without being consumed: “I am the God of your father, the God of Abraham, the God of Isaac, and the God of Jacob.” God is the God of the fathers, the One who had called and guided the patriarchs in their wanderings. He is the faithful and compassionate God who remembers them and his promises; he comes to free their descendants from slavery. He is the God who, from beyond space and time, can do this and wills to do it, the God who will put his almighty power to work for this plan.
§ 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 ,” 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”). 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.
§ 210 After Israel’s sin, when the people had turned away from God to worship the golden calf, God hears Moses’ prayer of intercession and agrees to walk in the midst of an unfaithful people, thus demonstrating his love. When Moses asks to see his glory, God responds “I will make all my goodness pass before you, and will proclaim before you my name ‘the LORD’ [YHWH].” Then the LORD passes before Moses and proclaims, “YHWH, YHWH, a God merciful and gracious, slow to anger, and abounding in steadfast love and faithfulness”; Moses then confesses that the LORD is a forgiving God.
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.
§ 213 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.
§ 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.
§ 216 God’s truth is his wisdom, which commands the whole created order and governs the world. God, who alone made heaven and earth, can alone impart true knowledge of every created thing in relation to himself.
§ 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. 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.
§ 221 But St. John goes even further when he affirms that, “God is love.” God’s very being is love. By sending his only Son and the Spirit of Love in the fullness of time, God has revealed his innermost secret: God himself is an eternal exchange of love, Father, Son and Holy Spirit, and he has destined us to share in that exchange.
§ 222 Believing in God, the only One, and loving him with all our being has enormous consequences for our whole life.
§ 225 It means knowing the unity and true dignity of all men: Everyone is made in the image and likeness of God.
Let nothing trouble you Let nothing frighten you Everything passes / God never changes Patience / Obtains all Whoever has God / Wants for nothing God alone is enough.

References: § 232
 § 3
 § 9
 § 8

§ 237
 § 254

§ 198

§ 199

§ 200

§ 205

§ 206

§ 207

§ 210

§ 213

§ 214

§ 215

§ 216

§ 218

§ 221

§ 222

§ 225