B. B. Warfield affirms this view: There is only one God, but in the unity of the Godhead, there are three eternal and co-equal Persons, the same in substance, but distinct in subsistence. (James Orr, ed., International Standard Bible Encyclopedia, vol. 5)
1. In matters of submission. The Son is subordinated to the Father, yet fully equal with the Father. The Spirit is subordinate to the Father and Son, yet fully equal with the Father and Son.
2. In matters of mission.
It was not the Spirit who sent his Son to this earth, but the Father. It was not the Father or Holy Spirit who became man and died on the cross, but the Son. It was not the Father or Son who came at Pentecost, but the Holy Spirit.
Robert Culver explains the orthodox understanding of these eternal relationships:
Two expressions have been traditionally employed to designate certain inner relations between the Father and Son, and the Father and the Son with the Spirit. These two expressions are the eternal generation of the Son by the Father and the eternal spiration (or procession) of the Spirit from the Father and the Son. They began to be employed about the time of Nicaea (AD 325). They expressed in scriptural language the idea that the Son and the Spirit were eternally with the Godhead. John 1:14 refers to our Lord as the “only begotten of the Father.”
John 14:16, 26 and 15:26 speak of the Spirit as “proceeding from the Father and the Son.” (The Living God, p. 96)
It should not surprise us that God, who is eternal, should be incomprehensible to temporal beings such as ourselves. There is nothing in our experience comparable to God. We have no common reference point to start from.