I am trying to understand the following. Let $E/\mathbb{Q}$ be an elliptic curve with complex multiplication given by the ring of integers $\mathcal{O}_K$.

We are given a fixed rational prime $p$ which is inert in $\mathcal{O}_K $. Then the image of the mod $p$ Galois representation $\overline{\rho}_{p,E}(G_\mathbb{Q})$ is conjugate to a subgroup of the normalizer of non-split Cartan.