The first characteristic of the pious is believing in the unseen.
The unseen is everything which cannot be perceived by your senses.
These senses are: The sight, the hearing, the smell, the touch and the taste.
The unseen refers to the things which are hidden from you. The existence of a thing is different from perceiving the thing. Therefore, it is untrue that everything that your senses do not perceive is non-existent. For example, bacteria cannot be perceived by the senses. The people alive during the period of revelation fell sick with high temperatures and illnesses such as malaria, typhoid and other diseases. And, they were cured without knowledge of germs nor of the causes of their illness. However, these germs were present, albeit them not knowing of them. Thus, ignorance of the existence of germs did not negate their existence.