How did Paul reconcile his Jewish heritage with the Gentile churches he planted around the Mediterranean sea? Ernst Käsemann (the beloved disciple of Rudolf Bultmann) wrote an outstanding essay titled Cry For Liberty in the Church's Worship, where he argues from Romans 8:26-27 that Paul dialectically embraced his Jewish-Christian heritage while simultaneously championing Hellenistic enthusiasm. Käsemann uses the familiar already/not-yet eschatological paradigm to describe how Paul already embraces spiritual enthusiasm such as speaking in tongues (glossolalia) in the Corinthian church as an inbreaking of the coming of the Lord Jesus Christ on the last day (parousia), but Paul has not-yet his Jewish heritage and was quick to cite his traditional Pharisee resume (e.g. Phil 3:4-7), circumcised Timothy (e.g. Acts 16:3), and brought his gospel first to the synagogues in the Jewish Diaspora. 

Others have used the already/not-yet paradigm to describe limited aspects of Paul's theology such as the Law-Gospel relationship or the delay of the coming of Jesus (parousia). Käsemann employs the already/not-yet to describe Paul's entire modus operandi where Hellenistic enthusiasm is the goal (teleos) of his theology and ministry. Käsemann rejects almost any hint of synchronism in Paul's already/not-yet nor is Paul choosing sides. 

"Nor is it any help to point out that the history of religion shows that the ancient world had strayed into devotional crises and doubts through syncretism and enlightenment. In Paul there is simply no sign of this at all, and only a few, at most, of his formulations can be so influenced." [1]

Käsemann argues that Paul's already/not-yet eschatology allowed him to champion and criticize both his Jewish heritage and the Hellenistic enthusiasm. So when Paul criticized his Palestinian Jewish opponents, he is doing so as an adherent to Judaism, and not as one who has abandoned Judaism. Likewise, when Paul criticizes speaking in tongues (glossolalia), he is doing so as a fellow enthusiast who also speaks in tongues, not as an opponent of enthusiasm. For example, Käsemann argues that Paul rejects the 'realized eschatology' in the Gentile churches because Paul believes enthusiasm is a spiritual derivative of future eschatology that will only be fully realized on the last day (parousia). So he does not reject enthusiasm but instead is orienting toward the eschatological end when all Christians would only then embrace spiritual enthusiasm.  

Here is an excellent longer quotation from Käsemann's essay of what I described:  

"The apostle's anti-Jewish polemic must not be primarily interpreted from the angle of the internal Jewish-Christian conflict. Paul, the Jewish Christian, came into conflict with those who belonged to the Palestinian tradition because he had become the champion of the enthusiastic Hellenistic churches which were growing up free of the Law. But his greatness lies in the fact that he did not throw himself without reservation into the arms of the new ideas which he had helped to produce and that he countered their excesses by means of his Jewish-Christian heritage. That is nowhere more evident than when he opposes to the 'realized eschatology' of the Hellenists, who were proud of their possession of the spirit, the spirit as the pledge and power of hope. In so doing he was by no means breaking new ground. It was in exactly this way that the Jewish-Christian community had already understood the spirit when they allowed it to proclaim, and in a representative sense to anticipate, the future basileia. The only new thing is the thematic unfolding of what was already implicit in the formula about the 'spirit of hope', in contrast to a sacramentally based insistence on the possession of the spirit." [2]

Romans 8:26-27 is the lynchpin of Käsemann's argument. 

"Likewise the Spirit helps us in our weakness, for we do not know how to pray as we ought, but that very Spirit intercedes with groanings too deep for words. 27 And God, who searches hearts, knows what is the mind of the Spirit, because the Spirit intercedes for the saints according to the will of God." (Romans 8:26-27 NRSV)

Käsemann argues that John Calvin and Martin Luther were wrong to reduce the scope of Romans 8:26-27 to the problem of prayer: "Finally, we must not, like Luther or Calvin before us, alter the problem as if it were the hearing of prayer" [3]. As I explained, Käsemann believes Paul's already/not-yet eschatology is not for this or that theological loci, instead, he uses these verses to demonstrate for Paul that the already/not-yet is all-encompassing. 

In Romans 8:26-27, Käsemann describes the spiritual groaning, not as a wordless prayer, but is describing the Holy Spirit establishing the church and directing it towards its end goal. Paul is embracing the Spirit-led enthusiasm, and rejecting an unguided enthusiasm, and this is precisely why Paul corrects the speaking of tongues (glossolalia) in 1 Corinthians 13-14, and rejects uninterpreted glossolalia. Paul is not forbidding glossolalia, he is establishing glossolalia praxis, not only for individuals but for the entire church.  And this is why the essay is titled Cry For Liberty in the Church's Worship, because the worship of the church is born from the Holy Spirit. Karl Barth once defined worship as the Church collectively repeating back what God has divinely revealed to the Church. Similarly here, Käsemann argues "prayer in Paul is never wordless" [4] and this includes the Spirit's groaning in Romans 8:26-27. 

To fully understand Ernst Käsemann's argument I highly recommend reading this essay. Cry For Liberty in the Church's Worship and is available in a collection of Käsemann's essays translated by Margaret Kohl titled Perspectives on Paul. (n.b. Margaret Kohl translated the supermajority of Jürgen Moltmann's works into English.) The entire collection is laudable, and although many of the essays are difficult to read and understand, I highly recommend them all, especially this one. 

Sources:

1. Käsemann, Ernst "Cry For Liberty in the Church's Worship." Perspectives on Paul, edit and trans. by Margaret Kohl, Fortress Press, 1978, p. 128.

2. Käsemann. Ibid. p. 125.

3. Käsemann. Ibid. p. 128.

4. Käsemann. Ibid.