Faith, simply put, is that we hold and accept to be true in the
absence of
evidence.
In itself, this is not problematic. Faith is an essential ingredient in our lives, whether we are religious or not. Even an atheist can have faith in things, even in something larger than he is, whether or not it may actually be God. Faith is what sustains us when we have nothing concrete on which we can rely, which happens from time to time.
But faith can also work against us. Like any other trait or behavior, faith can be a failing as well as a strength. This is seen most often when we observe others (or perhaps ourselves) who not only have faith in the absence of evidence, but faith in spite of the evidence. When this line is crossed, faith leads to dogma, closed thinking, and a polarized view of the world.
So, it's in our best interests to ensure that our faith, in whatever it is placed, is reasonable, realistic, and well-considered.