Liturgy is often defined simply as the work of the people. Understanding the Greek word helps us to better understand the reason the One Holy Catholic and Apostolic Church used and uses this word to describe the work of her members.
λειτουργία (leitourgía), from λειτ- (leit-), from λαός (laós, “people”) + -ουργός (-ourgós), from ἔργον (érgon, “work”) (the public work of the people done on behalf of the people)
This public service is done to sanctify ourselves and the world. The setting aside of sacred time and space brings the Heavenly Kingdom into our fallen world. Our Liturgy mirrors the divine reality. We become partakers of the Divine Energy through these works.
There is also a sense that the liturgy is a form of military service. Modernists try to downplay the warfare symbolism that permeates the writings of the Church. The Church on earth is the Church Militant. The rites, prayers and sacraments are the weapons we use to assault the passions, demons and old man. On this battlefield we plant seeds of virtue.
Liturgy is the constant reorienting of our life to the life giving Trinity: Father, Son and Holy Spirit.