linguistic:translation, information:classification
One God in three persons

In Trinitarian doctrine, God exists as three persons but is one being, having a single divine nature. The members of the Trinity are co-equal and co-eternal, one in essence, nature, power, action, and will. As stated in the Athanasian Creed, the Father is uncreated, the Son is uncreated, and the Holy Spirit is uncreated, and all three are eternal without beginning. "The Father and the Son and the Holy Spirit" are not names for different parts of God, but one name for God because three persons exist in God as one entity. They cannot be separate from one another. Each person is understood as having the identical essence or nature, not merely similar natures.

According to the Eleventh Council of Toledo (675) "For, when we say: He who is the Father is not the Son, we refer to the distinction of persons; but when we say: the Father is that which the Son is, the Son that which the Father is, and the Holy Spirit that which the Father is and the Son is, this clearly refers to the nature or substance".

The Fourth Lateran Council (1215) adds: "Therefore in God there is only a Trinity, not a quaternity, since each of the three persons is that reality – that is to say substance, essence or divine nature-which alone is the principle of all things, besides which no other principle can be found. This reality neither begets nor is begotten nor proceeds; the Father begets, the Son is begotten and the holy Spirit proceeds. Thus there is a distinction of persons but a unity of nature. Although therefore the Father is one person, the Son another person and the holy Spirit another person, they are not different realities, but rather that which is the Father is the Son and the holy Spirit, altogether the same; thus according to the orthodox and catholic faith they are believed to be consubstantial. "

Clarification of the relationships among the three Trinitarian Persons (divine persons, different from the sense of a "human self") advanced in the Magisterial statement promulgated by the Council of Florence (1431–1449), though its formulation precedes the council: "These three persons are one God and not three gods, for the three are one substance, one essence, one nature, one Godhead, one infinity, one eternity, and everything (in them) is onewhere there is no opposition of relationship ". Robert Magliola explains that most theologians have taken relationis oppositio in the "Thomist" sense, namely, the "opposition of relationship" is one of contrariety rather than contradiction. The only "functions" that are applied uniquely to the Father, Son, and Holy Spirit respectively in Scripture are the following: "Paternity" to the Father, "Filiation" (Sonship) to the Son, and "Passive Spiration" or that which is "breathed out", to the Holy Spirit. Magliola goes on to explain:

Because such is the case (among other reasons), Karl Rahner rejects the "psychological" theories of Trinity which define the Father as Knower, for example, and the Son as the Known (i.e., Truth). Scripture in one place or another identifies Knowing with each of the three Persons all told. Which is to say, according to the relationis oppositio, Knowing (in our example) does not define the Persons at all, but the Unity of God instead. (Scripture's attribution of Knowing to any one Person at any one time is said to be just "appropriated" to the Person: it does not really belong to that unique Person).

Magliola, continuing the Rahnerian stance, goes on to explain that the Divine Persons necessarily relate to each other in terms of "pure negative reference", that is, the three "Is Not" relations represented in the Scutum Fidei diagram are in each case a pure or absolute "Is Not". This is the case because the relationis oppositio clause disallows the Persons to "share", qua Persons, the unique role that defines each of them. Lest he be misunderstood, Magliola, in a subsequent publication, makes sure to specify that each of the three Persons, while unique as a Person, is nonetheless—because of the Divine "consubstantiality" and "simplicity"—the one Reality that is God.

There have been some different understandings of the Trinity among Christian theologians and denominations, including questions on issues such as: filioque, subordinationism, eternal generation of the Son and social trinitarianism.