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
Records found in Mexico
Nancy Casillas These are records found at Family Search