When the New Testament speaks of spirits, it recognizes the reality of a battle between the kingdom of God and evil spirits, a perspective depicted especially in the Gospels. Since there are both good spirits and bad, the church and individual Christians need to be able to discern the spirits (1 Cor 12:10, 14:2, 2 Cor 11:4, 1 Joh 4).
I think Karkkainen (my professor) makes an excellent point that has very impeccable application to the American church. He’s suggesting that we cannot afford to see the world through a materialistic-rationalist mindset which renders us oblivious to the spirits which are at work around us. In fact, the church that denies the presence and activity of evil spirits denies a large essence of Jesus’ very ministry– conquering the kingdom of Satan by the finger of God.