The Trinity, Father, Son and Holy Spirit, represent three relative relations in God; they do not indicate absolute or substantial distinctions within God. Thus, Peter Lombard in the Sentences explained:
These properties are designated by the names of the persons, namely, Father, Son, Holy Spirit, which are relative and are used in relation to each other because they denote relations. They are not accidental to God, but are immutable in the persons themselves from eternity, so that these titles are not relative, but are the very relations or notions in the things themselves, namely in the persons.
We must be careful in how we understand the three persons. For, as Lombard also explained, the term person is a convention which is used to help us understand what has been revealed to us about God:
But by some need, as Augustine said above, this noun is used in a metaphorical sense, so that we may say in the plural “three persons” when asked “three what”; there it does not signify essence, that is, the divine nature which is common to the three, but subsistences, or hypostases, according to the Greeks. 
When talking about the persons, therefore, we must not engage in any categorical mistakes. The terms are relative terms, and we must not confuse how we talk about each of the persons as a way of predicating relative distinctions as absolute qualities which indicate what God is. The Father, Son and Holy Spirit are all God. But by saying this, we must not reverse ourselves and say something like God is fatherhood, or God is sonship, or God is merely a spirit. The names which we use for the Trinity are used to represent their relations with each other (and in some ways, with us). Lombard therefore said that we could use other terms to indicate those relations:
THAT THERE ARE OTHER NAMES OF THE PERSONS DENOTING THE SAME PROPERTIES, NAMELY BEGOTTEN, BEGETTER, WORD, IMAGE. Here it is not to be passed over that, just as Father and Son and Holy Spirit are names of the persons and designate personal properties, so there are also other names of the persons, that is, names which signify the same persons and denote their properties, and the same ones as the aforesaid names [signify or denote]; and so [these names] are also used relatively: namely better, begotten, word, image. 
We use metaphors to represent the persons; these metaphors help us through analogy to indicate qualities which we associate with the persons of the Trinity as a way to express their relationships with each other. This, then is why we must not read into these designations their human counterparts. The analogies we use are useful, but there are distinct differences between the two analogues. For example, God not only is eternal, but he transcends human genders. The Father must not be understood like a human father who begets a child in time: the Father is without gender (and so is not to be said to be male), and the eternal begetting of the Son is not the same as human begetting, where two parents are required for the production of a child. Nor is the term Son to be understood as a gendered term, for in Christ there is also neither male nor female (cf. Gal. 3:28). In this fashion, Richard of St. Victor wrote:
And so, one ought to note above all that if in the deity who proceeds from one alone is correctly called the Son, and if he from whom alone the Son draws his origin correctly is called the Father, then we are reminded by these terms that a principal relationship undoubtedly exists in the divinity in a matter that absolutely cannot exist here in our nature. From these terms, I say, our carnal mind is compelled not to understand anything carnal about the divine generation, but ascent with our heart toward a higher understanding, and not to conclude rashly anything according to human measure about the mystery of such great profundity.
The Holy Spirit, moreover, must not be understood as the same as any other spiritual entity, as a non-physical being like other non-physical beings such as an angel. That is, there should be no equivocation between what we mean for spirit, sonship, and fatherhood in relation to our normal human experience of them with their analogues in the Trinity. Ficino, explaining this, said, “For such notions are [merely] intelligible rather than divine and are conceived only in the degree in which they can be captured by our [human] understanding.” The terms help us understand the relations which exist between the divine persons, but we must be careful and not use them to misconstrue our notions about God. As the persons are not to be understood in the same light as these terms are used for human relations, then they are not to be predicated to God, which is why Dionysius wrote: nor is It [God] Spirit (πνεῦμά) according to our understanding; nor Sonship (υἱότης), nor Paternity (πατρότη).
In the way the terms Father and Son are metaphors, so is the term Spirit a metaphor used to describe both the Holy Spirit and God in general (John 4:24). Scripture employs this term to negate a purely physical understanding for God. However, because Scripture suggests the existence of many kinds of spiritual beings such as angels, Dionysius made it clear we must see God as transcending what is normally meant by the term of spirit. The convention is employed because it helps show how God transcends materiality. But as there remains the possibility of many forms of such spirituality being, we must not let God be seen as their equal. While God is not material and therefore he can be said to be spiritual, he is not like any other being, as God is not a being, and so God is not to be confused as being like we conceived other spiritual beings. God, the first cause, transcends spiritual beings just like spiritual beings can be seen to transcend the material form of being. For this reason, we must not think of God as holding some form of spiritual substance.
Likewise, when we speak of God as our Father, we are following Scripture; but we must be careful in how we understand his parental relationship with us. God did not beget us. We are called to be sons and daughters of the Father as a way to indicate his love for us: good fathers and mothers love their children and take good care of them. God takes us in as if we are his children; he loves us like a parent, opening himself up to us so that we can become as it were adopted children of God. But this means he is not literally our Father: it is a mere metaphor. Similarly, there is no Sonship with God as God. We must not think that being called Son, there is a sense that there was a time in which the Son was not, before he was conceived. There is, however, a relationship within God where the person called Father has generated, not in time or space, nor through some union with some other persons, the person known as God the Son. There is therefore no absolute paternity or sonship in God, either in relation to us so that we see God as some sort of biological father for us, or in relation to the persons of the Trinity, where the Son is seen as a biological son with the Father, conceived with someone else.
For this reason, knowing the conventions and how they could be misinterpreted to establish a false notion for God, Dionysius denied God is Spirit like any other spirit, as well as denying Fatherhood and Sonship in God in any fashion, either in relation to us, or in relation to the intra-trinitarian reality. There are many errors which develop when we affirm these conventions as being absolutes in relation to God. Arius, for example, misconstrued the relationship between the Father and the Son because of his absolutizing of the conventions, leading him to believe the Son was born and established with time; St. Athanasius, therefore, explained that the terms Father and Son must not be understood the same way as they are understood in human generation. This, then, showed why such convention must not be absolutized, and apophatic theology, therefore, will take this seriously by denying the conventions, not because the conventions are wrong insofar as they are conventions, but because such conventions will always fail to meet the absolute truth. Only by denying the conventions can they be properly understood so as not to misconstrue God with human genders and biological functions. Thus, Anastasius explained, “Note that it is neither ‘spirit’ nor ‘sonship’ nor ‘fatherhood’ either as we do not understand them, or as we know them.” That is, we must not comprehend the persons as they are in themselves in their distinct relationships with each other through the conventions because our understanding of the conventions will lead us to misconstrue the reality implied by our conventions. Of course, in saying this, it must be clear: we do not deny the teaching of the Trinity. Rather, we must understand that what the teaching of the Trinity presents us is a higher truth through conventions which represent the best way which we can understand the immanent reality of God.
[IMG=Sainted glass picture of the Trinity from the Milan Cathedral by Picture by Giovanni Dall’Orto, [Attribution], from Wikimedia Commons]
 Peter Lombard, The Sentences: Book I. The Mystery of the Trinity. Trans. Giulio Silano (Toronto: Pontifical Institute of Medieval Studies, 2007), 141 [i-xxvi.2(106)].
 Peter Lombard, The Sentences: Book I. The Mystery of the Trinity, 136 [I-xxv.2(103)].
 Peter Lombard, The Sentences: Book I. The Mystery of the Trinity, 149 [I.xxvii.3(115)].
 Richard of St. Victor, On the Trinity. Trans. Christopher P. Evans in Trinity and Creation. Ed. Boyd Taylor Coolman and Dale M. Coulter (Hyde Park, NY: New City Press, 2011), 322-3.
 Marsilio Ficino, Mystical Theology in Marsilio Ficino: On Dionysius the Areopagite. Volume I: Mystical Theology and The Divine Names, Part I. trans. And ed. Michael J. B. Allen (Cambridge: Harvard University Press, 2015), 81.
 See St. Athanasius, “Discourse One Against the Arians” in NPNF2(4):322-23.
 Anastasius in A Thirteenth-Century Textbook of Mystical Theology at the University of Paris. trans. L Michael Harrington (Paris: Peeters, 2004), 105.
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