Why must the Son ascend before the Spirit may come? This question poses a special problem for the doctrine of inseparable operations (DIO). If the triune persons always act together, what explains the appearance of a substitution? It appears as if one divine player must leave the field/court to allow a substitute to come in. What’s going on here? Couldn’t Christ have sent the Spirit before his departure?
I’d like to suggest that a solution to this problem must be sought through the framework of the doctrine of the divine missions. Indeed, this apparent substitution bears directly on the doctrine of the missions, since it appears as if the mission of the Son ends with the ascension, while the Spirit’s mission commences at Pentecost.
I will only briefly indicate where I will be probing for an answer, leaving a fuller account for publication.
Taking Augustine and Aquinas as our guides, a mission of a divine person may be understood as the extension of a procession to include a created effect. And now in English: the coming forth of the Son/Spirit from the Father (procession) is extended towards the creature. The Son and the Spirit, who come out of the Father, pour out, so to speak, from the tranquil Trinity into the world. This may be expressed differently: bits of the world are drawn into the relations which exist within the life of the Trinity. The mission of the Trinity is to draw the world and to make it participate in the triune life (in various ways).
How does this understanding of the missions help us with our question? One implication of the doctrine of the divine missions is that a mission is the expression and extension of a procession. Thus, Jesus of Nazareth is the human expression of the Son’s eternal procession. Put differently, the created human nature of Jesus Christ has been hypostatically united to the eternal Son. The eternal procession of the Son is now expanded to include Christ’s human nature; the eternal Speaking by the Father of the Divine Logos now has an economic, created echo.
We’re close to an answer now; bear with me. The question seeks to correlate Ascension with Pentecost.
If the Son’s mission is an echo of his eternal procession, the Spirit’s mission will likewise be an echo of his own eternal procession.
But the Spirit’s procession is from the Son and the Father (according to Western Filioque). The Spirit proceeds from the love which the Son returns to the Father. The Spirit is the common gift between the Father and Son, a gift which in a certain way originates with the Father.
We are now in a position to give our answer. Just as in the immanent Trinity, the Spirit proceeds from the Son’s reciprocating the Father’s love, so economically, the mission of the Spirit presupposes the Son’s return to the Father in full submission (as human). Only this time, the Son’s returning the love of the Father happens in and through the Son’s created humanity. This is the obedience, love, and sacrifice of the second Adam.
For that reason, the mission of the Spirit is the completion of the overflow of Trinitarian life into creation. The mission of the Spirit is mediated through the humanity of Jesus Christ, which reciprocates in full obedience the love of the Father, being filled to overflow with the Spirit. For that reason Christ, the second Adam, became a life-giving Spirit (1 Cor 15:45).
There is no real substitution here. Christ has never left. He remains true to his promise to be ‘with us forever’. The humanity which the eternal Son in-dwelt, being fully submitted and sacrificed to God, now became itself a spring of living water (Jn 19:34). Christ in his very humanity became a re-spirator of the Spirit.
[Note: A fuller version of this argument will be published at some point in the near future. This is only a sketch of an argument.]