It’s used to display the blessed host to the public during a Procession or Benediction (a Roman Catholic or Anglo-Catholic devotion including the exposition of the eucharistic Host in the monstrance and the blessing of the people with it).
The Monstrance can also be called Ostensorium . Both names are derived from Latin words (Monstrance is monstrare and Ostensorium is ostendere) and they both mean “To show ” .
Christ has always wanted us to feel his presence in physical form which is why he instituted the sacrament of Eucharist, offering his body and blood as food under the sign of bread and wine and told his disciples to do this in memory of me.
During mass in the Catholic Church, the bread and wine is transformed into the body and blood of Christ as Christ has instituted. This transformation , called transubstantiation, means that the entire substance of the bread and wine is transformed into the Body and Blood of Christ, while the accidents alone (size, weight, color, flavor, etc.) remain. Once the transformation has occurred the Catholic believe that each and every host present during the Consecration is in real presence of Christ Body, Blood, Soul, and Divinity .
The blessed host is a sacred substance in catholic church because when blessed it becomes the body of Christ, it is treated with respect and reverence which is why it’s carried inside a monstrance during procession and when the host is not being exhibited it’s stored in a sacred vessel called the Tabernacle with a red light shining beside the tabernacle symbolizing the presence of Christ.
The Monstrance was first used in France and Germany in the 14 century, when the devotion to the blessed sacrament became popular.
The Monstrance have different designs, the portable ones are carried by a priest , while the fixed one are kept in a special place or Chapel known as chapel of Blessed Sacrament. For the portable monstrance, the preferred design is a sunburst on a stand, topped by a cross.
The monstrance is often made of silver-gilt or other precious material , and highly decorated. In the middle of the sunburst, the monstrance normally has a little round transparent glass the size of a Host, through which the Blessed Sacrament can be seen. Behind this glass is a round vessel made of glass and gilded metal, called a lunette, which holds the Host securely in place.
During the adoration of the blessed sacrament, the priest blesses the people by raising the monstrance containing the blessed host. This blessing is different and unique because it’s believed to be a direct blessing from Christ and not from the individual priest.
The adoration of the blessed sacrament dates back to the beginning of Christianity, when monastic hermits will keep the Blessed Sacrament over long period of time, by praying in the presence of the Blessed sacrament, venerating the body, blood, soul and divinity of our Lord Jesus Christ, which is known as Eucharistic adoration . As the monasticism become popular, the reserved Eucharist moved to a special container ( Tabernacle) and can be placed near the altar or sanctuary for people to pray and venerate it.
Pope Urban IV established the Feast of Corpus Christi in 1262 to honor the mystery of the presence of our Lord Jesus Christ’s Body, Blood, Soul, and Divinity in the Eucharist outside of the Mass. Eventually, a procession of the Eucharist in the monstrance became part of the feast such that a bishop or priest carries the monstrance and the laity and clergy follow him on a line through the parish roads and community returning to the church for a Benediction.