Christ gave his disciples the authority "to heal all manner of sickness and all manner of disease" (Matth. 10:1). They would apply oil to many patients and heal them (Mark 6:13).
This therapeutic power stayed, by wish of the Lord, privilege of the Church: "Is any sick among you? Let him call for the elders of the church; and let them pray over him, anointing him with oil in the name of the Lord. And the prayer of faith shall save the sick, and the Lord shall raise him up; and if he have committed sins, they shall be forgiven him" (James 5:14-15).
The mystery is conducted by the "presbyters of the Church". The elements of the mystery are chrismation by oil and praying "in faith"; it is not, that is, something magical, it requires participation. The salvation will be brought by the Lord with His propitiatory blood. And it will not only be corporal but spiritual too.
The remission of sins does not mean here substitution of the mystery of holy confession, because holy unction is combined with confession. It does not mean either that God is forced to provide corporal health. This is more, really, the mystery of the love of the Church towards the patient. With this mystery we entrust completely the brother to the Providence and to the love of God.
The first Church used to conduct this mystery. In the "Egyptian Disposition" of Hippolytus (d. 236), there is a brief blessing prayer for the consecration of the holy oil: "in order to consecrate this oil, give health, o God, to those in need and receiving this; in the same way you have anointed kings, priests, prophets, this way too provide health to all that taste it and are in need of it" (c.f. fr. Trebellas, Dogm., tome 3, p. 351).
In the prayer that was saved in the "Commands of the Apostles" (around 380 AD), the minister concludes: "give strength that gives health, that keeps sicknesses away, that makes demons run away, that drives away every bad action with the help of Christ ..." (Comm. of Apostl., H, XXIX, 2-3).
St. John Chrysostom commemorates those that "healed their ailments after having arrived with faith and in the right time and were anointed with oil" (Chrys., To Matth., Speech 32:6). At another point he adds that the priests have authority to forgive sins not only when they "rebear" us, but later on too. This father of the Church secures this authority with the verse James 5:14 For a more detailed analysis of the Mystery please go here.
EXPLANATION OF TERMS USED IN THE CHAPTER
mysteries = A mystery in the orthodox faith refers to the transmission of the invisible divine grace via visible signs. For instance, what the Roman Catholics call "the seven sacraments" are all mysteries, as well as the nativity of the Son from the Father and the procession of the Spirit from the Father and His mission from the Son are all mysteries. For more on this subject, visit this link.