John here informs his readers that they have a clear test as to the authenticity of a teacher or prophet. The test is not his behaviour, or his popularity, or his success. Instead it is doctrinal accuracy. John refers to a heresy that was infiltrating the church of his day. The wrong idea suggested that Jesus was not fully man, that he only seemed to be like a man. It was an attempt to explain the complexity of the person of Christ. Many were confused by this teaching and it became necessary for true believers to confess verbally what they believed about Jesus.
Why did John bring in this test? Does he want all his readers to become scholarly theologians, to have a Christian life that is mainly cerebral? Of course not! He has already explained in his letter how important it is that believers obey God’s commandments. John mentions this test because he knows that ignorance of basic doctrines is the greatest danger that a Christian faces and its existence gives the devil his easiest opportunity of making inroads into the Christian church.
There are two sets of people who are responsible for ensuring that this doctrinal knowledge is always understood, and each set has taken a vow regarding it. One set is the officebearers of the church who have taken vows in which they stated that they understood these doctrines and were thus able to protect the church. The other set is parents, who have the responsibility of teaching their children.
In a sense, what is in mind here is family protection. Officebearers protect the church family from error and parents protect their families from error. And they have to be proactive as well as reactive. There is little benefit in giving instruction to a church when an issue suddenly arises; they way to deal with such possibilities is to teach the faith constantly to one another whenever they have fellowship. Similarly, parents should fill their children’s minds with the truth and it will become the grid by which they assess what comes their way. But when a church or a family is without such a doctrinal framework, it is easy for the devil to deceive.