A good first step is to review exactly what the dogma of the Trinity is, and is not. The Catechism of the Catholic Church has a lot to say about the subject, but in my view these are some of the most important passages to help our understanding.
CCC253 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…
CCC254 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…
CCC267 Inseparable in what they are, the divine persons are also inseparable in what they do. But within the single divine operation each shows forth what is proper to him in the Trinity, especially in the divine missions of the Son’s Incarnation and the gift of the Holy Spirit.
In the Nicene Creed, we profess that Jesus is “the Only Begotten Son of God, born of the Father before all ages.” It is very important to understand the intent of this statement is not to imply that the Father somehow caused the Son to exist, or preceded him in time. The Father is not the “cause” of the Son and the Spirit; they along with the Father have existed for all eternity. “Before all ages” tells us we are not talking about an event in time. Rather, we are speaking to the relationship that has existed eternally between the three persons. “Born of the Father” is not an event closed by time, but a way of being within God himself. Paragraph 255 of the catechism speaks to this relationship:
CCC255 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….”
My favorite way to rather simplistically express what we believe about the Trinity is that there are three distinct persons who are so completely united in every thought, word, action and moment of being for all eternity, they are one God. I can’t remember now where I first read that, but it has remained with me and given me a small glimmer of understanding of something that is truly beyond our comprehension.
Along with our Catholic catechism we see very similar language in the statements of faith from other Christian groups who profess a belief in the Trinity. Some examples:
From the Southern Baptist SOF — “There is one and only one living and true God. ….The eternal triune God reveals Himself to us as Father, Son, and Holy Spirit, with distinct personal attributes, but without division of nature, essence, or being.”
From the Lutheran Book of Concord – “there is one Divine Essence which is called and which is God: eternal, without body, without parts, of infinite power, wisdom, and goodness, the Maker and Preserver of all things, visible and invisible; and yet there are three Persons, of the same essence and power, who also are coeternal, the Father the Son, and the Holy Ghost.”
From the Westminster Confession of Faith – “In the unity of the Godhead there be three persons, of one substance, power, and eternity: God the Father, God the Son, and God the Holy Ghost.”
From the 39 articles of Anglicanism – “There is but one living and true God, everlasting, without body, parts, or passions; of infinite power, wisdom, and goodness; the Maker, and Preserver of all things both visible and invisible. And in unity of this Godhead there be three Persons, of one substance, power, and eternity; the Father, the Son, and the Holy Ghost.”
From Smyth’s Short Confession of 1610 – “These three are not divided, nor separated in essence, nature, property, eternity, power, glory or excellency.”
From the Confession of Faith of The Evangelical United Brethren Church – “We believe the one God reveals himself as the Trinity: Father, Son and Holy Spirit, distinct but inseparable, eternally one in essence and power.”
The “Shield of the Trinity” is not intended to be a “schematic diagram” of the structure of God but is simply a visual device that contains the truths of the Athanasian Creed in a visual way and has been a helpful tool for many.