There is something infectiously attractive about a person who is very enthusiastic about what they believe, especially if they are willing to sacrifice everything for it. We tend to think that their sincerity is a mark of authenticity. It is a common impression given by successful sales people. The unseen logic is that because they are so committed, and they are nice people, then their product must be excellent.
 
Missionary work is not exclusive to humble servants of the gospel. The Pharisees were highly zealous advocates for what they believed; and some of them were missionaries for their cause. (The Apostle Paul was a travelling Pharisee missionary, enthusiastic to eradicate the Christian faith, before Jesus met with him - Acts 22:3-5). Jesus exposed their motivation: it was not from heaven but from hell.
 
The Pharisees were hypocrites, actors, not only to others but also to themselves ... they believed the lies they were spreading. Like people carrying a deadly virus they had no insight into the spiritual danger they had become. Their enthusiasm was deadly because their convictions were motivated by the devil (John 8:44). Jesus was training the disciples to become leaders in the Early Church, which would be in danger from 'wolves' masquerading as 'sheep' (Matthew 7:15). Their common characteristic would be that they would not accept that Jesus Christ was God in the flesh, and submit to Him (2 John 7).
 
It is the same today. Thousands of religions and philosophies have their missionary disciples. The church must not be gullible. Every spiritual influence must be tested against the Lordship of Jesus Christ. Church and family leaders must be particularly vigilant to guard the flock (Acts 20:28). The best way to protect against lies and liars is to practice and proclaim the truth. How many children would be spiritually protected if their parents read and explained God's Word to them each day. Do not believe every spiritually enthusiastic voice: check it out against God's Word and God the Son. And do not welcome any influence that is anti-Christ into your house (2 John 10).