on faith, take it

take (something) on faith

To accept something without further verifying or investigating, based on trust. You're right to be wary, but, in this case, I think we can take John's statement on faith. He has no reason to lie to us.
See also: faith, on, take

on faith, take it

Trust, accept without proof, as in I have no firm evidence that Bob's responsible for the errors-you'll just have to take it on faith . This idiom employs faith in the sense of "belief or confidence in something," a usage dating from about 1300.
See also: on, take