The Holy Mysteries:
The Eastern churches use the term “mysteries” to describe the sacred rites by which the Church perpetuates the saving action of Christ on earth. The Western church uses the term “sacrament”. Each term has a history and is filled with meaning. In the Eastern churches, mystery generally refers to the realm of the holy and to God's plan of salvation. With the eyes of faith we are able to appreciate the real presence of God in creation.
The process of being united to Christ is achieved through the "mysteries". In other words, Christ not only performed acts of divine power during His public life, but He continues His divinizing power in the mysteries that He instituted. Just as Christ used earthly things and gestures as instruments of divine power, so He provided that through the invoking of the Holy Spirit by the Church on water and oil and bread and wine we would have the means of sanctification.
Therefore, Baptism, Chrismation, Eucharist and the other sacred rites are called mysteries because they introduce us to the world of the holy which is incarnated in our visible world. They enable us with the eyes of faith to realize that "God is with us" and that His Spirit is available to us.
Our Divine Liturgy teaches us that during the service of the Eucharist, our earthly celebration mirrors the angelic liturgy in heaven. In other words, during the course of the Anaphora we are brought into sacred time and sacred space, the realm of mysteries. In the same way, when we celebrate baptism, Chrismation and the other mysteries, we also enter the world of sacred time and space and partake of holy things.