In my opinion, this is absolutely true. Because there are so many variables involved in getting a whole economy to run smoothly, it is impossible to know what effect a given action will have on the overall economy. Because there are so many variables, it is also impossible to know what caused a given outcome.
In other words, economists do not know what caused the financial crisis. If they had known, they would have spotted it coming. A few did, most didn't. Economists do not know what actions will get us out of the crisis either.
When a government puts a given policy in place, they have to go with their best guess as to whether it will work. Some economists will say they're right, others will say they're wrong, no one will know for sure.
It's not completely based on faith -- there are some things that are known, but the majority are not.