The Great Rite, in Wicca, is a symbolic reenactment of the union or marriage of the Goddess and God (heiros gamos). In some Traditions, this reenactment is performed by the High Priest and the High Priestess through physical sexual union. As a matter of fact, I recall several years ago when I had considered studying with a local Alexandrian coven where this literal reenactment of the great Rite was a mandatory part of the coven’s practices (that coven has since disbanded). Generally, most covens prefer a more symbolic form of reenactment through the use of the Chalice (Goddess) and Athame (God) during the cakes and ale portion of the ritual. The “sexual union” of the God and Goddess is achieved by the High Priestess holding the Chalice and the High Priest and High Priestess, together, grasping the Athame and plunging it gently into the ale or beverage contained within the Chalice with the appropriate words.
The Great Rite is actually the centerpiece of Wiccan ritual. It is the blessing which is intended to ensure prosperity to the participants through a symbolic act of fertility. In this sense, it is actually a form of sex magic (but without the actual physical sexual encounter). Legend has it that, in some covens, the High Priestess and the High Priest would actually engage in sexual intercourse in the fields as a way to “telling” the crops to be fruitful and fertile. This symbolic re-enactment of the Holy Marriage delivers us a paradox and a mystery; the God and Goddess lose Themselves in this physical Unions—which can be understood in very human terms—and, yet, they are still a Duality, a representation of the polarity of the Divine essence that is also found in humankind.
Wicca is a relatively new religion. As such, it lacks the cult of personality that some other religions may have. This causes Wicca to be more dependent upon experience and re-enactment of ancient European rites to lend it authenticity and authority. It serves as a tool to reconnect modern Wiccans with the ancient wisdom that so many seek in a world that appears to be spiritually adrift in this day and age.
In the last coven that my partner and I oversaw (the Circle of the Mountain Fire), we had a tradition during the Great Rite. As each person passed the chalice or the plate to their neighbor, we would say to the recipient, “thou art God” or “thou are Goddess” as an acknowledgement of the Divine within them.
 In a very real sense, this practice has its counterpart in the Christian faith system where [a] the Church is the Bride of Christ and [b] the legitimacy of the Church is conveyed through this marriage by allowing the Church to act as a part of this now “Holy Couple” as the Church is the “mistress” of this theological household. In secular terms, such a holy marriage would allow a king to rule; in Christian theology, this allows the Church to rule.