Roman Catholic Trinity Defined
When discussing the doctrine of the Trinity, we are immediately confronted with the fact that it is a doctrine clouded in mystery.
The dogma of the Holy Trinity
253 The Trinity is One. We do not confess three Gods, but one God in three persons, the "consubstantial Trinity". The divine persons do not share the one divinity among themselves but each of them is God whole and entire: "The Father is that which the Son is, the Son that which the Father is, the Father and the Son that which the Holy Spirit is, i.e. by nature one God." In the words of the Fourth Lateran Council (1215), "Each of the persons is that supreme reality, viz., the divine substance, essence or nature."
254 The divine persons are really distinct from one another. "God is one but not solitary." "Father", "Son", "Holy Spirit" are not simply names designating modalities of the divine being, for they are really distinct from one another: "He is not the Father who is the Son, nor is the Son he who is the Father, nor is the Holy Spirit he who is the Father or the Son." They are distinct from one another in their relations of origin: "It is the Father who generates, the Son who is begotten, and the Holy Spirit who proceeds." The divine Unity is Triune.
255 The divine persons are relative to one another. Because it does not divide the divine unity, the real distinction of the persons from one another resides solely in the relationships which relate them to one another: "In the relational names of the persons the Father is related to the Son, the Son to the Father, and the Holy Spirit to both. While they are called three persons in view of their relations, we believe in one nature or substance." Indeed "everything (in them) is one where there is no opposition of relationship." "Because of that unity the Father is wholly in the Son and wholly in the Holy Spirit; the Son is wholly in the Father and wholly in the Holy Spirit; the Holy Spirit is wholly in the Father and wholly in the Son."
Roman Catholic Trinity—A short version: Trinity, as espoused by the Roman Catholic Church, defines the One God of the Bible as a Single Divine Supreme Being composed of three distinct Persons (Father, Son, and Holy Spirit), who are co-eternal (without beginning, always existed alongside each other), co-equal (equal in position without hierarchy), indivisible (cannot be separated), consubstantial (an amalgamation of the same substance within a single "Being"), omniscient (all-knowing), omnipotent (all-powerful), omnipresent (everywhere present together). The Son exists through an "eternal generation,". which means that the Father eternally (continuously) generates the Son, and the Son is eternally generated by the Father. As the begetting of the Son is eternal, so the "procession"  of the Holy Spirit is also eternal. Learn More HERE.
"Now this is the Catholic faith: We worship one God in the Trinity and the Trinity in unity, WITHOUT either confusing the persons or DIVIDING THE SUBSTANCE; for the person of the Father is one, the Son's is another, the Holy Spirit's another; but the Godhead of the Father, Son and Holy Spirit is one, their glory equal, their majesty coeternal.INSEPARABLE IN WHAT THEY ARE, the divine persons are also inseparable in what they do.” (Catechism of the Catholic Church, Part one, The Profession of Faith, No.’s 266, 267)
The Catholic Church says, “The mystery of the trinity is the central doctrine of the Catholic faith. Upon it are based all the other teachings of the church.” — (Handbook for Today's Catholic, p. 11)
As Roman Catholic Graham Greene wrote, “Our opponents sometimes claim that no belief should be held dogmatically which is not explicitly stated in scripture . . . But the Protestant Churches have themselves accepted such dogmas, as the Trinity, for which there is no such precise authority in the Gospels,” — (Assumption of Mary, Life magazine, October 30, 1950, p. 51)
Furthermore, For Catholics Sunday is a day dedicated to the Trinity.
“Question: What is Sunday, or the Lord's Day in general?
Answer: It is a day dedicated by the Apostles TO THE HONOUR OF THE MOST HOLY TRINITY, and in memory that Christ our Lord arose from the dead upon Sunday, sent down the holy Ghost on a Sunday, &c. and therefore is called the Lord's Day. It is also called Sunday from the old Roman denomination of Dies Solis, the day of the sun, to which it was sacred.” — (The Douay Catechism of 1649, p. 143)
Hence, the true day of worship-the Seventh day Sabbath was replaced by the Catholics and it is on this very day (Sunday) that the Catholic church declares their dedication "TO THE HONOUR OF THE MOST HOLY TRINITY." This fact alone should cause every Adventists to give a very serious consideration.
Trinity—A Brief History
Few understand how the Trinity doctrine came to be accepted - several centuries after the Bible was completed! Yet its roots go back much farther in history.
Most people assume that everything that bears the label “Christian” must have originated with Jesus Christ and His early followers. But this is definitely not the case. All we have to do is look at the words of Jesus Christ and His apostles to see that this is clearly not true. Learn more HERE.
For further reading, please take a look at this short study, "What is the Doctrine of Trinity".