Why is CPT fundamental?

G. L"uders and W. Pauli proved the $\mathcal{C}\mathcal{P}\mathcal{T}$ theorem based on Lagrangian quantum field theory almost half a century ago. R. Jost gave a more general proof based on axiomatic'' field theory nearly as long ago. The axiomatic point of view has two advantages over the Lagrangian one. First, the axiomatic point of view makes clear why $\mathcal{CPT}$ is fundamental--because it is intimately related to Lorentz invariance. Secondly, the axiomatic proof gives a simple way to calculate the $\mathcal{CPT}$ transform of any relativistic field without calculating $\mathcal{C}$, $\mathcal{P}$ and $\mathcal{T}$ separately and then multiplying them. The purpose of this pedagogical paper is to deaxiomatize’’ the $\mathcal{C}\mathcal{P}\mathcal{T}$ theorem by explaining it in a few simple steps. We use theorems of distribution theory and of several complex variables without proof to make the exposition elementary.