Jesus teaches His disciples how to pray. Many have memorized this prayer and use it often. He then goes on to do battle with demons, which turns into a battle with His human opponents.
Their opposition to Jesus causes them to draw illogical conclusions about why Jesus has power over demons. The people who oppose Jesus the most are the religious leaders, so Jesus has harsh things to say about them. Jesus’ harshest words aren’t for people who seem like the worst sinners, they are for those who are experts on the Bible but don’t live according to it.
Jesus says something interesting about demons leaving a person and then coming back. If a person tries to overcome something evil, whether a demon or a sinful habit, they run the risk of it returning and being even worse than it was to begin with. The evil needs to be replaced by something more powerful. When Jesus died on the cross, He replaced our sin with His righteousness (1 Corinthians 5:21). When we trust Jesus, our tendency toward sin is replaced by the Holy Spirit (Romans 8:9, 10). Are you relying on just getting rid of evil in your life or are you relying on God’s provision of a replacement for the evil?