Both interplanetary space and Earth’s magnetosphere are populated by low-energy (≤300 keV) protons that are potentially able to scatter on the reflecting surface of the Wolter-I optics of X-ray focusing telescopes and reach the focal plane. This phenomenon, depending on the X-ray instrumentation, can dramatically increase the background level, reducing the sensitivity or, in the most extreme cases, compromising the observation itself. The use of a magnetic diverter, deflecting protons away from the field of view, requires a detailed characterization of their angular and energy distribution when exiting the mirror. We present the first end-to-end Geant4 simulation of proton scattering by X-ray optics and the consequent interaction with the diverter field and the X-ray detector assembly, selecting the ATHENA Wide Field Imager as a case study for the evaluation of the residual, soft-proton-induced background. We find that in the absence of a magnetic diverter, protons are indeed funneled toward the focal plane, with a focused non-X-ray background well above the level required by ATHENA science objectives (5 × 10‑4 counts cm‑2 s‑1 keV‑1), for all the plasma regimes encountered in both L1 and L2 orbits. These results set the proton diverter as a mandatory shielding system on board the ATHENA mission and all high throughput X-ray telescopes operating in the interplanetary space. For a magnetic field computed to deflect 99% of the protons that would otherwise reach the WFI, Geant4 simulations show that this configuration, in the assumption of a uniform field, would efficiently shield the focal plane, yielding a residual background level of the order or below the requirement.