The Pharisees were rule-followers. One of their favorite rules stated that people could not work on the Sabbath (Saturday). They had added rules on top of what God required just to make sure they didn’t accidentally work on the Sabbath. For example, you had a maximum number of steps allowed to be taken. Counting my steps all day sounds like work to me. Since Jesus and his followers were breaking one of these rules, the Pharisees were convinced that Jesus was God’s enemy. They explained Jesus’ miracles as having been done in Satan’s power. Jesus strongly condemns this kind of unbelief. If a person is going to reject a clear demonstration of God’s power and attribute it to Satan, there is no hope.
There are those today who are blinded by their strongly held beliefs which prevent them from seeing the power of God. Overcoming this kind of unbelief is not easy.