From The Visible Church, some good discussion on “Sacramentals”: “…anything set apart or blessed by the Church to excite good thoughts and to increase devotion, and through these movements of the heart to remit venial sin.” (p. 119). Specifically, the Sign of the Cross, the Cross, and the Crucifix:
The Sign of the Cross is the most important of the sacramentals, being a symbol of our deliverance from the power of Satan, and an emblem of God’s mercy manifested through the crucifixion of our Saviour on the cross of Calvary.
The words and the action form a summary of our faith. We say: “In the name” — not “names” — expressing thus the unity of God. We mention the three Persons, the Father, the Son, and the Holy Ghost, thus showing our faith in the Blessed Trinity. The cross itself, made with the hand, manifests our belief in the incarnation, death, and resurrection of our Saviour, and shows that we regard Him not only as God but as man — for unless he possessed a human nature He could not die. (p. 119-120).
The Cross is the most important of Catholic emblems. It symbolizes the redemption of mankind and our holy faith, because Jesus Christ, our Redeemer and our God, died on a cross.
Among many nations, in ancient times, crosses were used for the execution of criminals. But even among pagan nations the cross was held in religious honor. The most ancient form was the swastika, emblematic of the revolutions of the sun, and consequently of life. In Egypt and Assyria the cross typified creative power; the Egyptian gods are often represented holding the crux ansata, or cross with a handle, an emblem of the reproductive powers of Nature. In India, Mexico and Peru, crosses were in use with the same symbolic meaning. (p. 121).
The Crucifix. There is a difference between a cross and a crucifix. A cross becomes a crucifix only when it bears an image of our Lord’s Sacred Body. The word crucifix is from the Latin crucifixus, fixed to a cross.
The tablet bearing the letters I N R I at the top of the crucifix is called the title, and these letters are the initials of the words Jesus Nazarenus, Rex Judaeorum (Jesus of Nazareth, King of the Jews) — the letters I and J being the same in ancient Latin.
On some crucifixes a skull and bones are shown at the foot — reminding us that Golgotha, the place of the crucifixion (Mount Calvary), signified a skull (from its shape or because it was a burial place — or possibly from the fanciful legend that in the hold dug for our Lord’s cross was found the skull of Adam!) (p. 123).