Великое освящение храма в честь архангела Михаила в скиту Оптиной Пустыни
In the tradition of the Eastern Orthodox Catholic Church, the Consecration of a church (Εγκαινια Ναου) is the service of sanctification and solemn dedication of a building for use as a church. The consecration of a church is a complex service that is filled with profound symbolisms. Many biblical elements are taken from the Old Testament: the Consecration of the Tabernacle (Exodus 40) and of the Temple of Solomon (1 Kings 8; 2 Chronicles 5-7).
While the consecration encompasses the whole church, the ceremony centers around the holy altar and holy altar table in particular. As salvation for an Orthodox Christian is union with Christ, called Life in Christ, the center of this Life in Christ is the holy altar, the consecration of a church is, in effect, the baptism and chrismation of the church.
In the early days of Christianity when the Church was heavily persecuted, the Christians met in underground burial places where they celebrated the Eucharist on the graves of martyred saints. After the Church was recognized this custom was continued by placing relics in the altar table during the consecration of the church. This is a reminder that the Church was built on the blood of the martyrs and their faith in the Lord.
After the bishop has entered the church, he continues into the altar. In the altar he places the diskos on the altar table. There he removes the relics and places them in a small box. The bishop then pours holy chrism over the relics, symbolizing the union between our Lord and his martyrs. With prayers and the reading of Psalm 145, the bishop then places the box with the relics in a cavity in the altar table where it is sealed in with a wax/mastic that contains fragrant spices as were used by Joseph of Arimathea and Nicodemus to anoint our Lord’s body before his burial. In this, the holy altar represents Christ’s tomb.
– Excerpts from Orthodox Wiki