Gelasian Theory or the Theory of the Two Swords

Gelasius I on Spiritual and Temporal Power, AD 494

The Gelasian tradition is about the autonomy that each of the great powers, the Temporal and the Spiritual, has within its own sphere. Gelasius affirmed there was a hierarchy between these two powers or "kingdoms": Imperium versus Sacerdotium.

Pope Gelasius I in writing to the emperor Anastasius in 494 declared that there were two forces that ruled the world, the sacred sovereignty of the priesthood and the executive power of the prince. Both were God- given, and while the priestly authority was greater, inasmuch as it guided even the emperor's soul as that of a son of the church, yet the priesthood ought to obey the emperor in matters of public, secular interest.

This is known as the Two Sword Theory. Gelasius asserted that the temporal sword is hierarchically lower than the spiritual sword. 

Letter of Pope Gelasius to Emperor Anastasius on the superiority of the spiritual over temporal power:

http://www.fordham.edu/halsall/source/gelasius1.asp

There are two powers, august Emperor, by which this world is chiefly ruled, namely, the sacred authority of the priests and the royal power. Of these that of the priests is the more weighty, since they have to render an account for even the kings of men in the divine judgment. You are also aware, dear son, that while you are permitted honorably to rule over human kind, yet in things divine you bow your head humbly before the leaders of the clergy and await from their hands the means of your salvation. In the reception and proper disposition of the heavenly mysteries you recognize that you should be subordinate rather than superior to the religious order, and that in these matters you depend on their judgment rather than wish to force them to follow your will.

If the ministers of religion, recognizing the supremacy granted you from heaven in matters affecting the public order, obey your laws, lest otherwise they might obstruct the course of secular affairs by irrelevant considerations, with what readiness should you not yield them obedience to whom is assigned the dispensing of the sacred mysteries of religion. Accordingly, just as there is no slight danger in the case of the priests if they refrain from speaking when the service of the divinity requires, so there is no little risk for those who disdain - which God forbid -when they should obey. And if it is fitting that the hearts of the faithful should submit to all priests in general who properly administer divine affairs, how much the more is obedience due to the bishop of that see which the Most High ordained to be above all others, and which is consequently dutifully honored by the devotion of the whole Church.

Extracted from

James Harvey Robinson (1904). Readings in European History. Wildside Press LLC. pp. 72–. ISBN 978-1-4344-7084-3.

http://en.wikipedia.org/wiki/Pope_Gelasius_I

Later theological development

The Medieval conception of society was as Ecclesia, i.e. the community of Christians as members of the Christians Church both Lay and Spiritual. Society could be divided into two authorities, the Spiritual [Sacerdotium] and the secular [Imperium or Lay], which have to work together in harmony.

Bernard of Clairvaux, ep. 86; ep. 256,  De Consideratione III, I, I;

Patrologia Latina Tomus 184 https://goo.gl/43PwwF
https://archive.org/stream/bernarddeclirvau00bernuoft#page/82/mode/2up  https://goo.gl/XHRRSv

THE 'DE CONSIDERATIONE' OF ST. BERNARD OF CLAIRVAUX AND THE PAPACY IN THE MID-TWELFTH CENTURY: A REVIEW OF SCHOLARSHIP
Author(s): ELIZABETH KENNAN
Source: Traditio, Vol. 23 (1967), pp. 73-115
Published by: Cambridge University Press
Stable URL: http://www.jstor.org/stable/27830827

John of Salisbury, ep. 44. 

Patr. Lat. 199, col. 27. 4-5 https://goo.gl/TRLpUf

Unam Sanctam

The papal Bull on papal supremacy AD 1302

In this bull, pope Boniface teaches that there is only one Kingdom, the Church, and that the Church controls the spiritual sword, while the temporal sword is controlled by the State. The temporal sword is hierarchically lower than the spiritual sword. The superiority of the spritual sword allows the Church to influence politics and society at large.

Among other things the Bull lays down the dogmatic proposition of the unity of the Church, and of the necessity of belonging to it for eternal salvation.

The pope is supreme head of the Church, and a duty of submission to the pope by catholics necessarily arises, if they are to belong to the Church and if salvation is to be attained.

Kirsch, J.P. (1912). Unam Sanctam. In The Catholic Encyclopedia. New York: Robert Appleton Company

http://www.newadvent.org/cathen/15126a.htm

Church and State in Christian History

David Knowles

Journal of Contemporary History

Vol. 2, No. 4, Church and Politics (Oct., 1967), pp. 3-15

Published by: Sage Publications, Ltd.

Stable URL: http://www.jstor.org/stable/259819

The Two Swords controversy and the roots of modern political theory

DE Clark - 1989
www.dtic.mil/dtic/tr/fulltext/u2/a218350.pdf
https://goo.gl/KhT5oj