One can base their knowledge on two things: experience and faith. It’s easy to say “I know it because I saw it!” Why is it so indescribably hard to say “I know, because I believe!” One can’t say he has experienced everything he knows. There is knowledge that has no other proof than the belief in them. Those peaces of knowledge are based on faith. Easiest example is learning a foreign language. Thatcher teaches a new expression and the student has to believe that what the teacher has just said means just that what the teacher said it means. If the student would learn the language by double of triple checking everything, it would take considerably longer to learn the new language.
It is not possible to experience everything. Even if someone would base all their knowledge on their experience and things that they have seen, then that persons knowledge will be very limited.
If all knowledge would be based on tests and experiences and some more tests then the development of that society would stop.
Thus we need faith to progress faster in our lives.