The holy symbol of the Hammerites is of course a hammer. There are three types seen in the Thief Series;
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.
The smaller quest related holy symbol from TDP/TG "Return to the Cathedral", where it is needed to perform the Ritual of Consecration to aid the spirit of Brother Murus. It must be forged in St. Tennor's factory and blessed in Renault's Lunar Pool.
Same item can be found in T2 mission "Eavesdropping". Located in one of the chests down in the catacombs, it looks exactly like the unblessed holy symbol from TDP/TG although this one seems to be useless.
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.