They base their literal interpretations in biblical doctrine on Mark 16:18, primarily, among other scripture. This passage is quite known by many.
Mark, chapter 16:16-18 ~ Verse 16: He who believes and is baptized will be saved; but he who does not believe will be condemned. Verse 17: And these signs will follow those who believe: In My name they will cast out demons; they will speak with new tongues; Verse 18: They will take up serpents; and if they drink anything deadly, it will by no means hurt them; they will lay hands on the sick, and they will recover.” NKJV
Note, this is where the "laying-on of hands" belief in faith-healing harkens back too also. (Also drinking strychnine.) Giving poor innocent, uneducated people hope that the right pair of untrained human hands will cure leprosy, demons, cancer, Alzheimers, polio, ALS, MS, MD, infertility, paralysis, and all manner of things with no present cure.
Note, also, as they have stated quite frankly on the show,...if their Lord commands them (I take it they know when this happens) to take up the copperheads, timber rattlers, and cottonmouths, and they do not do it, they believe they will burn in everlasting hellfire.
Absolutely fascinating.
They believe that everything in their lives and religion is driven by an authority greater, higher, and above human law.
And if you do not believe as they do...well...same fate.