The holy symbol of the Hammerites is of course a hammer. There are three types seen in the Thief Series;

DromEd Object Model holyham
The red and gold "Holy-Hammer", seen in TDP/TG and T2, only has a useful purpose in the TDP/TG mission "Undercover", where a bridge will extend across the Talisman chamber if the hammer is in Garrett's inventory.

DromEd Object Model hammis11
The smaller quest related holy symbol from TDP/TG "Return to the Cathedral", where it is needed to perform Brother Murus' consecration rites. It must be forged in St. Tennor's factory and blessed in Renault's Lunar Pool.

TDS loot sec holysymbol
The still smaller quest related holy symbol from TDS mission, "St. Edgar's Eve", where it is required to lower the reliquary cage containing the Builder's Chalice.