First of all, the original promise of Our Lady of Mt. Carmel to St. Simon Stock
regarding the brown scapular had nothing to do with the small scapular many Catholics commonly wear today. At the time, the Carmelite order was trying to establish itself in Europe and was facing opposition. Many in the Church thought the Carmelite hermits who had moved into Europe from the Holy Land should join the current religious orders in Europe rather than maintain their own order. There was also a lot of contention over the Carmelites' habits, which were remarkably different from the habits of the European orders of the time.
St. Simon Stock, an early superior general of the Carmelites, sought to adapt the Carmelites' hermitic life to one similar to the mendicant friars such as the Franciscans and Dominicans, but while retaining the Carmelite identity. According to tradition, the Blessed Virgin appeared to St. Simon holding a large, full-body brown scapular similar to the one Carmelites wear today and gave it to St. Simon as the habit of the Carmelite order. It was to this
brown scapular that the promise was attached. Later, smaller scapulars suitable for wearing by laypeople were approved by the Church as a sacramental.
If we are to take Our Lady of Mt. Carmel's promise in a strictly literal fashion, we'd have to say that the wearer of the scapular must wear the Carmelite habit. But this obviously isn't the case. And, in truth, the promise shouldn't be taken strictly literally. The promise is dependent upon the wearer's desire to meet his Catholic obligations, to do his best to lead a virtuous life, and to work with the graces made available to him through the sacraments of the Church and through the brown scapular. If he refuses to do that, his wearing of a brown scapular is little more than pious superstition. Certainly it is not
in any way a guarantee of heaven.
These days, many in the Carmelite order continue to promote wearing the brown scapular but downplay the promise that "Whosoever dies wearing this scapular shall not suffer eternal fires" precisely because of the superstitious means by which too many promote it. The scapular I currently wear, which was made by local Carmelite nuns, does not use that particular phrase but instead says:
Journey to Carith
Behold the sign of salvation; put on the Lord Jesus Christ.
by Peter-Thomas Rohrbach