Belief in the written and oral Torah tradition

I studied in a yeshiva, and I was exposed to blatant lies from rabbis from whom I learned a lot of Torah in a variety of contexts, both in everyday contexts and in halakhic contexts, in which those rabbis hide the halacha or say that the halacha is different than they think.
So, how can I believe their words that Moses received the Torah from Sinai, and that if he received it, his words did indeed reach us and that things that were not in accordance with the spirit of a particular period were not omitted and others were added?