This God taught Israel to say ‘The Lord our God is One.’ There are distinctions of course. The NT writers, and Christ Himself, noted that OT prophets like David and Isaiah, when ‘in the Spirit,’ were party to conversations within the Godhead from the deepest past of pre-temporal ‘time’ (anthropomorphism if ever there was one). The fathers referred to this as prosopological exegesis, in which the prophet, using language scripted by the Holy Spirit, speaks in the prosopon, the character, the persona of a divine participant in the drama of redemption (See Matthew Bates, The Birth of the Trinity). The prophet could, in the Spirit, hear conversations before and after his own location in history. In Psalm 2 David hears the Father say to the Son at from eternity, ‘Today I have begotten you.’ As the church meditated on this and other texts, it realized that the Father (as the principle) generates, the Son is generated and the Father and the Son spirate the Holy Spirit. These actions are not understood to be distinct in themselves, but with respect of the persons they are distinct; the action by which the Father generates the Son, and the action by which the Father and Son spirate the Holy Spirit is nothing other than the pure act of God (actus purus). Within the Trinity the Father and the Son are ontologically related to one another in that the Father is only the Father in relation to the Son and the Son is only the Son in relation to the Father. Aquinas was building on the Greek fathers and Augustine’s teaching when he conceived of the persons of the Trinity as subsistent relations, that is, they subsist or exist as who they are only in relation to one another. The Holy Spirit proceeds from the Father as the One in whom the Father begets the Son in love, and He proceeds from the Son as the One in whom the Son loves the Father who has begotten Him. The Holy Spirit is the product of the Father’s love for the Son, and the Son’s love for the Father.
Augustine and Aquinas’ thoughts on the Trinity must be understood in the light of the prior canonical pronouncements of Nicea and Constantinople and not as an alternative. In my view John of Damascus’ closely argued chapter in the Exact Exposition is helpful in this regard. http://venerable-bede.blogspot.ru/2016/06/john-of-damascus-exact-exposition.html