The notion of Purgatory, by which sinners could expiate their sins by prayer, including the prayers of others and the intercession of saints and holy figures, was a firmly held Catholic tenet adopted by the Council of Trent in the face of Protestant disbelief.
In the 17th century it enjoyed a great vogue in the New World and several popular saints, including the Virgin Mary in various guises, were portrayed as divine intercessors on behalf of the suffering souls.
In this particular version the intercessor is Our Lady of Mt. Carmel, flanked by Teresa of Avila, the reformer and founder of the Discalced Order, and St Joseph, to whom the Carmelites were especially devoted.
The crowned Virgin displays the Carmelite insignia and holds a brown scapular—a form of devotional apparel special to the Order. Together with the rosary, the scapular was popularly associated with indulgences and salvation which accounted for much of its appeal in colonial Mexico.
The naked souls of kings, popes and bishops are arrayed in the flames of Purgatory, all wearing scapulars, while an angel above displays a copy of the Sabbatine Privilege—a controversial dispensation allegedly granted to observant Carmelites by the 14th century Pope John 22.
According to legend the Pope had a vision of Our Lady granting that through her special intercession She would personally deliver the souls of Carmelites from Purgatory on the first Saturday after their death ("Sabbatine" means Saturday), providing they fulfill certain conditions including the wearing of the brown scapular.
https://purgatorysouls.blogspot.com/