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.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.