The oldest catholic church records from all parishes were maintained at the Basilica of St. John the Baptist in St. John's. Baptisms there start at 1802 and marriages at 1793. Best of all...records at the basilica are indexed. The above info is found on familysearch.org. You can go through the records of Catholic parishes on familysearch.org. Several start earlier than 1843.
I'm enclosing a link that should open a list that includes the city of St. John's and access to the indexed records at the Basilica.
https://familysearch.org/ark:/61903/3:1 ... cc=1939435Perhaps you have already tried this, if so apologies.
L. Moss