Within your question is a kind of mysterious truth. Keep in mind there are 2 world-views. One is spiritual. The other is "material". The man who is material only sees what is visable and so he does not believe in anything spiritual. He believes himself as scientific, rational and logical and sees the spiritual man as anti-scientific and superstitious (and stupid).
The spiritual (Christian) man knows HE HIMSELF has a spirit. He knows God is a spirit. If he is a Christian... he has accepted the spirit of God (Jesus), to lead and guide his life.
When the material man reads the Bible, he only looks for logical rational truths... but all he sees is superstition and
anti-logical-nonsense. But the SPIRITUAL man reads the Bible looking for spiritual truth. The Bible is full of spiritual truth. The Bible must be read from a spiritual mindset, not a physical worldview mindset. Jesus explains this in Mark 4:12
"seeing they may see and NOT PERCEIVE, and hearing they may hear and NOT UNDERSTAND."
The material-world-man can begin to understand spiritual things when he truly wants to change his heart. NOT when he reads the bible angrily and doubtfully.
So... the Christian is not talking about "spiritism" or "psychics" or "ghosties." He is speaking about the spirits in men and of God's spirit that might be invited in. And possibly of the spirits of angels or demons (fallen angels)... or of Satan (another fallen angel). Those are the spirits Christians are concerned with. A Christian can "see" these spirits at work in other people and in nations. Not actually SEE them.. but it is a perception or "knowing" that comes from spiritual discernment.