1 John 4:6
 We are from God. Whoever knows God listens to us; whoever is not from God does not listen to us. By this we know the Spirit of truth and the spirit of error. (ESV)
You can tell what people believe by discovering who they listen to. As the Apostle John continues to instruct the church he lets them know that one of the major distinctions between true believers and those who are enemies of God is whether or not they are willing to listen to apostolic teaching. John is an Apostle and speaks for the validity of the teaching of the Apostles.
Following the teaching of the Apostles is foundational to our faith. Those who listen to apostolic teaching are from God. If we go back to the very beginning of the church we will find that the Apostles teaching was something very important to those new converts. And if we are truly part of the same church we will also have that same devotion. Take a look with me at the Book of Acts, which tells the story of the beginning of the Church.
 And they devoted themselves to the apostles' teaching and the fellowship, to the breaking of bread and the prayers. (ESV)
If you go back and look at our verse in 1 John 4 we are told that listening to the Apostles' teaching is how we know the Spirit of truth. The important application of this truth is that the Holy Spirit will NEVER contradict the truth that he has already spoken through his Apostles. When you hear someone claim to be speaking from the Holy Spirit, test their words against the truth of the Apostles' teaching.
Do you follow the Apostles' teaching? If you do not listen to it, John has some sobering indictment of who you belong to. If you do listen to the Apostles' teaching, you will be able to detect the false teaching that is not from the Spirit of truth.