According to Bronsted-Lowry Theory an acid is a proton or hydrogen ion donor while a base is a proton or a hydrogen ion acceptor.
In this case, both HCl and HC5H5N+ are acids as they are donors of hydrogen ions. HCl is an acid to the forward reaction while HC5H5N+ is a acid to the reverse reaction.
On the other hand, C5H5N and Cl- are bases, they are acceptors of hydrogen ions. Cl- is a base in the reverse reaction while C5H5N is a base in the forward reaction.
