The Catechism says the following:
2111 Superstition is the deviation of religious feeling and of the practices this feeling imposes. It can even affect the worship we offer the true God, e.g., when one attributes an importance in some way magical to certain practices otherwise lawful or necessary. To attribute the efficacy of prayers or of sacramental signs to their mere external performance, apart from the interior dispositions that they demand, is to fall into superstition (cf. Mt 23:16-22).
2138 Superstition is a departure from the worship that we give to the true God. It is manifested in idolatry, as well as in various forms of divination and magic.
Now, with this definition as our guide, we see that the scapular would be magical or superstitious if we believed that the mere ownership of the scapular or its mere presence on one's person was enough to ensure that person a place in heaven. But, the scapular is not a good luck charm and it is not enough to simply have it around your neck when judgment comes. It is what is implied by the wearing of the scapular that makes all the difference.
The promise of Mary to St. Simon Stock that "whosoever dies wearing this scapular shall not suffer eternal fire" extends only to those who wear the scapular as a sign of their devotion to the Blessed Mother and to living a good, Christian life of prayer and holiness. When you put on the scapular, you set yourself apart as someone who wishes to live as Mary lived, and as someone who humbly places himself under her mantle.
When we pray the "Hail Mary" we say at the end: "Holy Mary, Mother of God, pray for us sinners, now and at the hour of our death. Amen." That becomes the petition of every person who faithfully wears the scapular. He wears it with faith that Mary will pray for his soul at the time of Judgment. And if he has lead a good life, he will not suffer eternal fire.
The scapular itself also encourages and maintains the devotion that it's presence is supposed to symbolize. We feel it throughout the day, when the front or the back drops down too far, when the squares gently scratch us or pat against our bodies as we run. All of these are small reminders to live as Mary lived. We show our love to Mary by kissing the scapular, and we implore her intercession every time we clutch it in fear or sorrow. All of this is very good and pious Catholic practice.
For more on the scapular, see the following articles:
- An Explanation of the Sabbatine Privilege
- Fr. William G. Most: The Brown Scapular
- Different Kinds of Scapulars
- The Five-Fold Scapular
- New Advent: Scapular
- Rite for the Blessing of and Enrollment in the Scapular of the Blessed Virgin Mary of Mount Carmel
- The Scapular Devotion
- Permission by the Holy Office to Wear the Scapular Medal
- Scapular of Carmel a Treasure for the Church
- Around the Year with the Trapp Family: Our Lady of Mount Carmel
- Spirit of Carmel: Brown Scapular