How can the authenticity of the prophetic hadiths be proven to someone who believes in God but denies the prophethood of Muhammad (peace be upon him), taking into account the varying authenticity of hadiths over time?

"Whoever does not believe in the prophethood of Prophet Muhammad, peace be upon him, is a disbeliever. And whoever does not believe in his existence at all has an ناقص (incomplete/deficient) intellect, because his existence is a historical truth, widely transmitted and agreed upon by both Muslims and disbelievers. As for casting doubt on the Prophetic Sunnah by claiming its distortion and the presence of weak narrations within it, this is a flimsy specious argument. The Sunnah is a proof by consensus, and scholars have preserved it, recorded its authentic parts, and distinguished them from the weak ones with precise scientific rules."