Relation in the Trinity, St. Thomas
“Distinction in God arises only through the relation of origin. However, a relation is in God not as an accident inherit in a subject, but it is the divine essence itself; therefore it is subsistent just as the divine essence subsists. So, just as Godhead is God, the divine Fatherhood is God the Father, who is a divine person. Hence ‘divine person’ signifies a “relation as subsisting”. This means a relation in the manner of a substance which is a hypostasis subsisting in the divine nature; though what is subsisting in the divine nature is just the divine nature itself.”
St. Thomas Aquinas, S. Theol.
‘A relation in the manner of a substance which is also a hypostasis subsisting in the divine nature’ – this is St. Thomas’s mature and considered understanding of ‘person’ as applied to the Father, Son. and Spirit.
“It is the Father who anoints, and it is the Son who is anointed by the Spirit. The Spirit is the unction.” – Irenaeus (c. 180 A.D.
“Who will not say that there is one God? Yet, he will not on that account deny the [eternal] Economy.” – Hippolytus (c. 205 A.D.