take something on faith

take something on faith: 

to accept or believe something on the basis of little or no evidence. 

• Please try to believe what I'm telling you. Just take it on faith. 
• Surely you can't expect me to take a story like that on faith.

to believe someone or something without proof. 

• You will have to take it on faith that the information I am asking for is really important.
  

... http://idioms.thefreedictionary.com/take+on+faith