The Mysteries, or Sacraments, of the Church
Sacraments of the Orthodox Church are ceremonies embodying tangible signs of the Divine Grace inscribing in an invisible way, through a supernatural energy on the souls of the recipients, their redeeming effects, and regenerating them in the life of Christ.
The Greek word is 'mysterion' from which the words mystical and mysticism come from. Its root is the verb 'myo' which means to close the eyes for the purpose of protecting them from an extraordinary vision of deity.
The sacraments of which there are Seven were established by Christ directly (Baptism and the Eucharist), or through his Apostles who instituted them under Christ's teaching with the guidance of the Holy Spirit (Marriage, Confession, Unction and Ordination).
Since they were initiated by Christ Himself, or by the implicit request of Christ, they are necessary for our salvation. They were instituted as a means of transmitting Grace based on the needs of the believers. This is not to limit the action of the Holy Spirit as it can surely act outside of these ceremonies. But, Grace can only be received through Baptism and the Eucharist which were directly established by Christ.
Sacraments have both an outward and inward aspect. John Chrysostom says, "We see one thing and we believe another." The Sacraments of the church are both symbols and bearers of Divine Grace.
A Sacrament to be valid must be performed by a Priest and cannot be performed outside the Church. The Priest serves as an instrument and is not the responsible principal of the sacrament. As Saint John Chrysostom says, "the Father, Son, and Holy Spirit perform while the priest lends his tongue and extends his hand."