“The distinctions between the Persons within the essence of the Godhead and the distinctions between their respective duties and work are explained theologically by the eternal generation of the Son by the Father and the eternal procession of the Holy Spirit from the Father and the Son….
“By the eternal generation of the Son is meant that eternal act whereby the Father communicates or makes common the divine essence to the Son—the eternal sonning of the Son….
“The essence is indivisible and cannot be divided into three persons; it can only be “made common,” not divided or parceled out to individual personalities, so that the whole of the divine essence (the unity) is in each of the three personalities (the trinality)….
“Similar to eternal generation, procession refers to that eternal act whereby the Father and the Son make the divine essence common to the Spirit—the eternal spirating of the Spirit…. [T]he external, temporal mission of the Spirit is predicated on the internal distinctions of the Trinity, which in the case of the Spirit goes back to His eternal procession from the Father and the Son…. [B]oth the Father and the Son send the Spirit. The Father sends the Spirit in Christ’s name….
“Eternal generation and procession describe the theological processes whereby the eternal, undivided essence of God subsists wholly, indivisibly, simultaneously, and eternally in Father, Son and Holy Spirit. The Father generates the Son and together with the Son spirates the Spirit. It is always in that order theologically. Both generation and procession constitute an eternal, divine act….
“The doctrines of eternal generation and procession also account theologically for the three individual persons in the Godhead and for their evident administrative and functional order. It explains the relations and activities within the one, undivided, all-pervading, unceasingly circulating common essence or deity, relative to matters both internal and external to the Godhead….
“The internal activity is the eternal trinalizing of the essence or the resultant Trinitarian distinctions of the First, Second, and Third Persons. The First Person is the Father, the ingenerate paternal subsistence of the divine essence. The Second Person is the Son, so called because the divine essence is made common to Him by generation/filiation. The Third Person is the Spirit, so called because the divine essence is made common to Him by spiration/procession….
“As for the external activity, it refers to the transitive work of the Trinity via the functional order of the economy existing between the three Trinitarian persons. The Father does not generate some of His own fatherhood or His personal paternity to the Son, nor do the Father and Son do so for the Spirit. However, it is still the person of the Father, not the essence, which generates the person of the Son (not essence)…. All three persons issue from the essence of deity; therefore, all are equally and eternally God in the Ontological Trinity (i.e., having to do with being or essence). But the eternal generation of the Son by the Father and the eternal procession of the Spirit from the Father and the Son form the structure of the Economic Trinity (having to do with function)….
“Both generation and procession are eternal acts. There is no thought of origin, nor any notion of temporal succession. It is an eternally simultaneous action by which the Father, Son, and Holy Spirit subsist as co-equal, consubstantial, and co-eternal persons of the triune God. There is never any thought of one member existing previously to the others, nor of one ever being non-existent. Neither is there any idea that the relationships within the Godhead of Father, Son, and Spirit have been anything but eternal (e.g., that at some point the Second Person became the Son).”
–Rolland McCune, A Systematic Theology of Biblical Christianity, I:292-294