The missions of the Son and the Spirit - each of them is a means to the other

Which of the two missions [of the Son and the Holy Spirit] is the means and which is the end? Thinking in terms of redemption accomplished and applied, it is easy to consider the Spirit's work as the means of delivering the Son's work. In that case the Son does the the thing itself - reconciling God and man - and the Spirit serves his work by applying it to individual lives. Pentecost, in this view, happens in order to fulfill and extend Calvary and Easter. On the other hand, when we consider the intimacy of spiritual fellowship that the indwelling of the Holy Spirit involves, it begins to look as if all God's ways lead up to the sending of the Spirit.

If God's goal is to dwell among his people, the atonement was a necessary step to make indwelling possible. The temple of human nature had to be cleansed to make it ready for the Spirit's indwelling. So the Son by incarnation prepared one perfect human temple, his own body, for the Spirit to be present in. And by atonement the Son purified the other temples, preparing them to receive the Spirit on the basis of the Son's finished work. "Christ redeemed us from the curse of the law by becoming a curse for us ... so that we might receive the promised Spirit through faith" (Gal 3:13-14), Calvary and Easter, in this view, happen in order to make Pentecost possible. If we have to ask whose work is the means and whose is the end, our answer must be either that the question is badly formed, or that each of them is a means to the other. The Spirit serves the Son by applying what he accomplished, and the Son serves the Spirit by making his indwelling possible. Both the Son and the Spirit, together on their twofold mission from the Father, serve the Father and minister to us.

(pp.148-149, Fred Sanders, Embracing the Trinity: Life with God in the Gospel)