All major seforim were written with ruach hakodesh

 Rav Menashe Klein (7:160): Look at Divrei Chaim (Y.D. 2:105) concerning a teacher who insulted the honor of the Ohr HaChaim and said that the sefer wasn’t written with ruach hakodesh. After much discussion he concludes, “That in truth even in our days there are true sages who are not influenced by earthly matters and they have ruach hakodesh as we see stated explicitly in the Moreh Nevuchim and Ramban. Therefore not only did the Ohr HaChaim clearly compose his sefer with ruach hakodesh but all authors even in our generation if they are worthy of it compose their works with ruach hakodesh. That means that their wisdom and understanding is in agreement wth the Truth of the Torah. This is mentioned in the gemora with Rabbi Eliezar. It is also stated as a practical halachic ruling in Takfo Cohen (C.M. 25:123-124) where he says that one can not say “kim li” (use a minority opinion) against the ruling of the Shulchan Aruch because all of its rulings were written with ruach hakodesh. Therefore a teacher who denies the ruach hakodesh of the Ohr HaChaim is an apikorus because he doesn’t believe the gedolim who testified on the Ohr HaChaim that he was deserving of ruach hakodesh. Thus this teacher is a denier in the principle of ruach hakodesh and ridicules the words of the Talmud mentioned before and it is good that you did not leave your sons in his hands. “ See these words which are like firery coals. Therefore concerning this shochet, I say that if he doesn’t believe that the words of the Mishna Berura were composed with ruach hakodesh and therefore are in agreement with the truth of the Torah – he is a heretic and denier of the foundations of our faith. It is not only this shochet but all schochtim who do not believe in this their shechita is unkosher.