Who first understood the "authorities" in Romans 13 as spiritual, not secular, authorities? In his forthcoming book, Freedom's Progress, libertarian Gerard Casey describes several different ways of interpreting and applying Romans 13:1–7.  One approach in particular I had never encountered before:


that the "authorities" (ἐξουσίαις) of verse 1, often rendered "governing authorities," should be understood as spiritual or ecclesiastical authorities
that the "sword" (μάχαιραν) of verse 4, usually understood to be a physical sword, actually should be understood spiritually, as in the "sword of the spirit" (cf. Ephesians 6:17)


Here I'm not focused on the relative merits of this interpretation, but on its origin.  While not all in the early church were particularly pro-government, I haven't found anyone interpreting Romans 13:1–7 in the way Casey describes here.  I've also checked The Politics of Jesus by Anabaptist John Howard Yoder, and his treatment of Romans 13 doesn't match this either.

Who then first interpreted Romans 13:1–7 in this way, seeing the "authorities" and "sword" as spiritual in nature?
C.E.B. Cranfield, in his Critical and Exegetical Commentary on the Epistle to the Romans (vol. 2), attributes the first modern reference to this interpretation to Martin Dibelius (approximately 1909), but notes that it tends to be associated with Oscar Cullmann. He goes on to summarize the other supporters of this approach:


  It is, of course, clear and agreed that the civil authorities are referred to. What has been the subject of a considerable amount of dispute is whether there is in ἐξουσίαις a double reference&mdash;to the civil authorities and also to angelic powers thought of as standing behind, and acting through, the civil authorities. The suggestion of the double reference goes back (as far as modern times are concerned) to M. Dibelius, Die Geisterwelt im Glauben des Paulus, Göttingen, 1909. Though Dibelius later abandoned it,1 it was taken up by K. L. Schmidt,2 G. Dehn,3 K. Barth,4 O. Cullmann,5 and a number of other scholars.6 It has come to be specially closely associated with Cullmann, and it is he who has borne the brunt of the attacks upon it, in some of which a mood of impatience and irritation has sometimes been noticeable. Such adjectives as ‘grotesque’, ‘eccentric’ and ‘fantastic’ have been used of it, and it has often been very summarily dismissed....
  
  A useful account of the negative reaction to this interpretation has been given by [Clinton D.] Morrison.7 
  
  [Citation for entire quote]8


The Hermeneia commentators on Romans point to a later proponent of this interpretation as the first reference to translating ἐξουσίαις ὑπερεχούσαις as a reference to angelic powers (or more specifically, the angelic powers behind the earthly rulers), Karl Ludwig Schmidt (1937), also noting some of the same scholars listed by Cranfield:


  According to Lutz Pohle, Römer 13, 89, the first elaborations of the angelic interpretation were developed by Karl Ludwig Schmidt, “Das Gegenüber von Kirche und Staat in der Gemeinde des Neuen Testaments,” Theologische Blätter 16 (1937) 1–16, and Günther Dehn, “Engel und Obrigkeit: Ein Beitrag zum Verständnis von Römer 13, 1–7, ” in E. Wolf, ed., Theologische Aufsätze, Karl Barth, zum 50. Geburtstag (Munich: Kaiser, 1936) 100–109. See also Dehn’s later study, Vom christlichen Leben. Auslegung des 12. und 13. Kapitels des Briefes an die Römer, Biblische Studien (Neukirchen) 6–7 (Neukirchen-Vluyn: Neukirchener, 1954); the most influential advocate of this interpretation was Oscar Cullmann, The State in the New Testament (New York: Scribner’s, 1956; London: SCM, 1957, 1961) 95–114, followed by Clinton D. Morrison, The Powers That Be: Earthly Rulers and Demonic Powers in Romans 13:1–7, Studies in Biblical Theology 29 (London: SCM; Naperville: Allenson, 1960), and others.9


James D.G. Dunn indicates10 that an analysis of the meaning of ἐξουσία in patristic literature has been conducted by Karl Hermann Schelkle.11



Footnotes

1 ‘Rom und die Christen im ersten Jahrhundert’, in Sitzungsberichte der Heidelberger Akademie der Wtissenschaften, philosophische-historische Klasse (Heidelberg) 1941–42, Abhandlung 2 (1942), p. 7, n. 2.

2 ‘Zum theologischen Briefwechsel zwischen Karl Barth und Gerhard Kittel’, in Theologische Blätter (Leipzig) 13 (1934), cols. 328–34, and ‘Das Gegenüber von Kirche und Staat in der Gemeinde des Neuen Testaments’, in Theologische Blätter 16 (1937), cols. 1–16.

3 ‘Engel und Obrigkeit’, in E. Wolf (ed.), Theologische Aufsätze Karl Barth zum 50. Geburtstag, Munich, 1936, pp. 90–109.

4 Church and State.

5 Christ and Time, London, 1951; The State in the New Testament.

6 For names see C.D. Morrison, The Powers that Be: earthly rulers and demonic powers in Romans 13:1–7, London, 1960, p. 25, n. 2; W. Affeldt, Die weltliche Gewalt in der Paulus-Exegese: Röm 13:1–7 in den Römerbriefkommentaren der lateinischen Kirche bis zum Ende des 13. Jahrhunderts, Göttingen, 1969, p. 30.

7 Morrison, op. cit., pp. 40-54.

8 C. E. B. Cranfield, A Critical and Exegetical Commentary on the Epistle to the Romans, International Critical Commentary (London; New York: T&amp;T Clark International, 2004), 656–660.

9 Robert Jewett and Roy David Kotansky, Romans: A Commentary, ed. Eldon Jay Epp, Hermeneia&mdash;a Critical and Historical Commentary on the Bible (Minneapolis, MN: Fortress Press, 2006), fn. 57 on p. 787.

10 James D. G. Dunn, Romans 9–16, vol. 38B, Word Biblical Commentary (Dallas: Word, Incorporated, 1998), 757, 760.

11 Schelkle, K. H. "Staat und Kirche in der patristischen Auslegung von Röm 13:1–7." Zeitschrift für die neutestamentliche Wissenschaft 44 (1952–53), 223–36.