According to Adi Shankaracharya, Vedas consist of three parts-
Karma kand (rituals like yajna, nitya karma etc.)
Upasana kand (praying to devas etc.)
Bramha vidya (denunciation of world, sameness of Atman and Brahman etc)
Out of these three, first two have results relative to world because one does them believing one is merely a Jiva having certain desires. The third one gets rids of that ahamkar to acquire never ending bliss.
The procedure of second and third are clear to me. Second one requires shraddha and vishwas because Upasana is incomplete without it. My question is regarding the first one, does it also require faith, belief etc for it to give results?
For example one may do yajna to bring rain as described in Veda speculating whether it would work or not. He neither has belief nor disbelief in Veda. Would he get desired result? I would prefer sources from Shruti rather then Smriti supporting the answer. Adi Shankaracharya commentary would also suffice.