You have to know the name of the church to find a record. Open the file and then go directly to the Church as they are listed. Some have records going back to the 1500's. For example, I found Juan Casillas y Cabrera record of his legitimacy and as son to Martin Casillas by browsing in the files listed under Santa Catarina y Martir, and Bautimsimos de Hijos Legetimos. According to the index, this record was not available on-line, but it is. If you have a church name and approximate date you can find the records in the Distrito Federal Catholic Church Records, by scrolling back and forth.
Nancy Casillas
Saint Clair, Michigan