Sacramentals have been part of the Catholic Church in various ways from the very beginning. They are known as extensions of the seven sacraments and naturally flow from them.
Broadly speaking, sacramentals can be any number of actions or blessings that the Church has instituted over the years. The Catechism of the Catholic Church explains how sacramental blessings can be invoked over "persons, meals, objects, and places" (CCC 1671). These blessings call down God's grace upon a particular individual or object and ask for lasting spiritual protection.
One object of the Church that is among the oldest known sacramentals is the "Agnus Dei" disc. This is a disc of wax with the figure of a lamb impressed upon it. Historically these discs were worn around the neck and were made from the previous year's Paschal candle. They were originally created on Holy Saturday morning and distributed to the people on the following Saturday.
The tradition dates to around the 5th century, and later the pope was more intimately involved with the sacramental. It became a reserved blessing of the pope, who consecrated these pieces of wax during the first year of his pontificate and every seven years after that. It is believed that Pope Pius XII was the last reigning pontiff to bestow such a blessing.
The sacred wax was a constant reminder of Christ's Easter victory. According to various papal writings, those who wore it were instructed, "that at the sight or touch of the Lamb impressed on these waxen discs, the faithful should be inspired to dwell on the Mysteries of our Redemption recalled by this symbol, be moved to praise, venerate, and extol God’s goodness to us, hoping to obtain pardon for their sins, and to be cleansed from all stain of sin."
Below is a prayer for those who wear an Angus Dei sacramental that summarizes the spiritual disposition that the piece of wax was supposed to cultivate in the person wearing it. The prayer can still help us today to meditate on that saving action of the Passion, death and resurrection of Jesus and how that event should influence our lives.
My Lord Jesus Christ, the true Lamb who takest away the sins of the world, by thy mercy, which is infinite, pardon my iniquities, and by thy Sacred Passion preserve me this day from all sin and evil. I carry about me this holy Agnus Dei in thy honor, as a preventative against my own weakness, and as an incentive to the practice of that meekness, humility, and innocence which Thou hast taught us. I offer myself up to Thee as an entire oblation, and in memory of that sacrifice of love which Thou didst offer for me on the cross, and in satisfaction for my sins. Accept this oblation, I beseech Thee, O my God, and may it be acceptable to Thee in the odor of sweetness. Amen.