Personification is when something which is not human is given human characteristics. If you have a pet dog, think of the dog speaking to you, but with WORDS. Since dogs don't speak with words, but humans can, the dog is being seen as having the human ability to speak. As for your proverb, fear is given the human ability to knock as if it could stand and pound on the door while faith is given the human ability to open the door and give an answer. Neither one has the human ability to do this in real life; therefore, this proverb is a good example of the use of personification.
fear cant knock the door and faith cand answer. knocking the door and answering are abilities that only human beings can do so fear and faith are personalised