Can anyone explain me what does the statement mean: "the BRST symmetry is a non-linear symmetry, so the BRST is also a symmetry at the quantum level"? What does "at the quantum level" mean? In general I have a symmetry in the classical field theory action and then when I quantize the theory I get a Ward-Takahashi Identity. Is this identity the symmetry at the quantum level? I don't think so since it is a consequence of the non-linearity of the symmetry. Can anyone clarify this?