take sth as the gospel truth

take / accept sth as the gospel truth (to believe something is undeniably true) — верить как Святому писанию; принимать на веру; безусловно верить чему-л.

Example: My aunt believed that pouring a bit of salt on the floor would make her back stop aching. In fact, she accepted it as the gospel truth!

[gospel truth]
[article of faith]