What was the idiom for something that we take without a doubt?

We don't say it quite like Bulgarian. Instead in English we "take his word for it", as in: "I am taking his word for it". The idiom can be expanded to include "at face value", as in "to take his word at face value".

For example: I said to my friend, "The American told me that in the United States you should tip the waiter after paying the bill." My friend replied, "But we don't usually tip in Bulgaria; why should I tip in the United States?" I replied, "I don't know, but I am taking his word for it."

In the above situation, the first person is inclined to believe the American and has taken his statement (words) "at face value", even though the friend "has his doubts". Cascabel refers to "...Good as Gold" which is a contraction of the saying: "his word is as good as gold", which implies expertise in the field.

take [something] as gospel

Free dictionary:

To believe that something is absolutely true without any hesitation or reservations.

When we're growing up, we take what our parents tell us as gospel.

The beloved professor's opinions on the author are taken as gospel by his students.