http://www.theworkofgod.org/Library/Apologtc/R_Haddad/Course/Book1.htm#THE BLESSED TRINITY
“The doctrine of the Trinity is really a disguised form of polytheism – worshipping three gods in one!”
The Blessed Trinity is three divine Persons in one undivided God. It was Jesus Christ Himself who revealed the Blessed Trinity. Sacred Scripture clearly testifies to the existence of three distinct Persons in the one God:
“Then God said: Let us make man in our image, after our likeness…So God created man in his own image, in the image of God he created him…” (Gen. 1, 26-27);
“Then the Lord God said, See, the man has become like one of us, knowing good and evil…” (Gen. 3, 22);
“…Come, let us go down, and confuse their language there…” (Gen. 11, 7);
“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, and teaching them to obey everything that I have commanded you. And remember, I am with you always, to the end of the age” (St. Matt. 28, 19-20);
“And when he came up out of the water, immediately he saw the heavens opened and the Spirit descending upon him like a dove; and a voice came from heaven, Thou art my beloved Son; with thee I am well pleased” (St. Mark 1, 10-11).
This last verse most clearly reveals the First and Second Persons of the Blessed Trinity. Jesus Christ coming out of the water and His Father’s voice from heaven. The Holy Spirit, the Third Person, is revealed as a dove, descending from heaven onto Christ.
Jesus Christ as the Second Person of the Blessed Trinity is equal to the Father:
“Behold, a virgin shall conceive and bear a son, and his name shall be called Emmanuel, which means, God with us…” (St. Matt. 1, 23);
“In the beginning was the Word, and the Word was with God, and the Word was God”
St. John 1, 1).
The Word was with God in the beginning, and the Word was God. Two divine Persons, yet only one undivided God.
“The Father and I are one“ (St. John 10, 30);
“…the Father is in me and I am in the Father” (St. John 10, 38);
“…Whoever has seen me has seen the Father” (St. John 14, 9).
The Holy Spirit, Who proceeds from the Father and the Son as the Third Person of the Blessed Trinity, will be a living teacher of truth:
“And I will ask the Father, and he will give you another Advocate (Holy Spirit)…” (St. John 14, 16);
“…the Holy Spirit, whom the Father will send in my name, will teach you everything…” (St. John 14, 26);
“When the Spirit of truth comes, he will guide you into all the truth; for he will not speak on his own, but will speak whatever he hears, and he will declare to you the things that are to come” (St. John 16, 13).
To sin against the Holy Spirit is to sin against God:
“…why has Satan filled your heart to lie to the Holy Spirit and to keep back part of the proceeds of the land?…You did not lie to us but to God!” (Acts 5, 3-4).
“And there are three who give testimony in heaven, the Father, the Word, and the Holy Spirit. And these three are one“ (1 John 5, 7 [Douai]).
St. Clement of Rome, Letter to the Corinthians (C. 98 A.D.):
“Do we not have one God, one Christ, and one Spirit of Grace poured out upon us? And is there not one calling in Christ?”
The Martyrdom of St. Polycarp (C. 155 – 157 A.D.):
“…In this way and for all things I do praise you, I do bless you, I do glorify you through the eternal and heavenly High Priest Jesus Christ, your beloved child: through whom be glory to you with Him and with the Holy Spirit, both now and through ages yet to come. Amen.”
St. Theophilus of Antioch, To Autolycus (181 A.D.):
“The three days before the luminaries were created are types of the Trinity: God, His Word, and His Wisdom.”
St. Irenaeus of Lyons, Against Heresies (C. 180 A.D.):
“If any one, therefore says to us, ‘How then was the Son produced by the Father?’ we reply to him, that no man understands that production, or generation, or calling, or revelation, or by whatever name one may describe His generation, which is in fact altogether indescribable. Neither Valentinus, nor Marcion, nor Saturninus, nor Basilides, nor angels, not archangels, nor principalities, nor powers (possess this knowledge), but the Father only who begat, and the Son who was begotten. Since therefore His generation is unspeakable, those who strive to set forth generations and productions cannot be in their right mind, inasmuch as they undertake to describe things that are indescribable.”
St. Ambrose of Milan, Hexameron (Post 389 A.D.):
“But let us consider the course of our own creation. He says: ‘Let Us make man to our image and to our likeness.’ Who says this? Is it not God, who made you?…To whom does He say it? Certainly not to Himself, for He does not say ‘Let Me make’ but ‘Let Us make.’ Nor to the Angels, for they are ministers; and servants can have no partnership in the operation of the master, nor works with their author. It is the Son to whom He speaks, even if the Jews will not have it and the Arians fight against it…[And it is the Son] who is the image of God the Father, the Son who always is and who was in the beginning.”
St. Augustine of Hippo (+430 A.D.), De Trinitate, Bk. 7, Ch. 4:
“For that which must be understood of persons according to our usage, this is to be understood of substances according to the Greek usage; for they say three substances, one essence, in the same way as we say three persons, one essence or substance.”
Catechism of the Council of Trent (1566):
Since nowhere is a too curious inquiry more dangerous, or error more fatal, than in the knowledge and exposition of this, the most profound and difficult of mysteries…
But these truths which should not be made the subject of too subtle investigation, when we recollect that he who is a searcher of majesty shall be overwhelmed by glory. We should be satisfied with the assurance and certitude which faith gives us that we have been taught these truths by God Himself, to doubt whose word is the extreme of folly and misery. He has said: Teach ye all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost; and again, there are three who give testimony in heaven, the Father, the Word, and the Holy Ghost; and these three are one.
Catechism of the Catholic Church (1992):
No. 234: The mystery of the Most Holy Trinity is the central mystery of Christian faith and life. It is the mystery of God in himself. It is therefore the source of all the other mysteries of faith, the light that enlightens them…
No. 237: The Trinity is a mystery of faith in the strict sense, one of the mysteries that are hidden in God, which can never be known unless they are revealed by God. To be sure, God has left traces of his Trinitarian being in his work of creation and in his Revelation throughout the Old Testament. But his inmost Being as Holy Trinity is a mystery that is inaccessible to reason alone or even to Israel’s faith before the Incarnation of God’s Son and the sending of the Holy Spirit.