...Paul theologized about the Spirit of God in light of his preconceived Jewish eschatology. The Spirit is given by God (1) as an assurance of the resurrection of the body, (2) as an attestation of Jesus’ messiahship, and (3) as a confirmation of God’s preordination of the Messiah’s sacrificial death. 

These points are dynamically interrelated and articulated in context to the major eschatological realities of Jewish apocalypticism (i.e., the day of YHWH, the resurrection of the dead, the judgment, and the Parousia of the Messiah), which are generally referenced without definition. Without any explicit statements concerning the redefinition or realization of these events by the Spirit in this age, it seems most plausible that the Spirit simply confirmed for Paul that Jesus was indeed the Jewish Messiah, and his Parousia would indeed actualize Paul’s apocalyptic hopes. In this way, Paul sought to disciple his Gentile hearers into the first-century Jewish narrative—“you, although a wild olive shoot, were grafted in” (Rom. 11:17)—by means of the Spirit.  - John P. Harrigan