  In this paper we present a complete computation of the Cosmic Microwave
Background (CMB) anisotropies up to third order from gravitational
perturbations accounting for scalar, vector and tensor perturbations. We then
specify our results to the large scale limit, providing the evolution of the
gravitational potentials in a flat universe filled with matter and cosmological
constant which characterizes the Integrated Sachs-Wolfe effect. As a byproduct
in the large scale approximation we are able to give non-perturbative solutions
for the photon geodesic equations. Our results are the first step to provide a
complete theoretical prediction for cubic non-linearities which are
particularly relevant for characterizing the level of non-Gaussianity in the
CMB through the detection of the four-point angular connected correlation
function (trispectrum). For this purpose we also allow for generic initial
conditions due to primordial non-Gaussianity.
