Think of a similar word "faithful". Being faithful means you can be relied upon. When one is described as being faithful, there is normally some form of evidence behind it, whether tangible or not. And thus "faith" is the evidence of things unseen. You therefore can have "faith" in someone that is "faithful" because you know it to be true. You saw it, you felt it, you know it, ect.
Think of a similar word "faithful". Being faithful means you can be relied upon. When one is described as being faithful, there is normally some form of evidence behind it, whether tangible or not. And thus "faith" is the evidence of things unseen. You therefore can have "faith" in someone that is "faithful" because you know it to be true. You saw it, you felt it, you know it, ect.