Monoenergism (a Greek loanword meaning "one energy") is a particular heretical teaching within the field of study known as Christology about how the divine and human relate in the person of Jesus Christ. The teaching arose in an initial attempt by Patr. Sergius I of Constantinople and emperor Heraclius to form an agreement after the Council of Chalcedon between the Chalcedonians and non-Chalcedonians. Specifically, Monoenergism teaches that Jesus Christ had two natures but only one energy, whereas orthodoxy teaches that Jesus Christ acts through two energies, divine and human, generally called Dyoenergism.
Discussed at the synod of Garin at Theodosiopolis, present day Erzurum, in 622 as a solution to reconciling the non-Chalcedonian Church of Armenia, the doctrine of Monoenergism enjoyed support for several years by the Churches of Constantinople, Alexandria, and Antioch. However, the monk Sophronius in Palestine was not convinced of the soundness of the doctrine and championed the doctrine of Dyoenergism, that is Jesus Christ had two natures and two wills. Using the authority of his new position when he became patriarch of Jerusalem in 634, Sophronius challenged the validity of the doctrine of Monoenergism.
Countering Sophronius' position, Patr. Sergius, in 635, received the endorsement of Pope Honorius I of Rome that all discussions concerning one or two energies should cease and agreed with the doctrine of Monoenergism. Meanwhile, Sophronius presented his position in this Epistola synodica (Synodical Letter) that attempted to show that the Monoenergism doctrine was inconsistent with orthodoxy. His position was that it was a bastardized form of Monophysitism. Soon former supporters of Monoenergism were busy finding flaws and inconsistencies in the monoenergistic proposal. As a result, Patr. Sergius and emperor Heraclius abandoned monoenergism as a doctrine.
Yet, they did not give up as Monoenergism was a precursor to Monothelitism a further development of the Monophysite position in the Christological debates.
The Sixth Ecumenical Council at Constantinople in 680-681 adopted Dyoenergism as church doctrine and at the same time rejected Monoenergism.