1 John 2:21
 I write to you, not because you do not know the truth, but because you know it, and because no lie is of the truth. (ESV)
Not everyone who claims to know the truth actually does. The false teachers in the church of the Apostle John's day had claimed that they alone had special knowledge for salvation. They were wrong. The Apostle John writes to bring clarity to the turmoil that was going on in the church. To those who were faithfully following the Savior John writes that he is writing to them because they know the truth. What an encouragement. They had been spiritually bullied and told that they were lacking in their knowledge, but the reality is they were the ones who knew the truth.
How did the believers in Jesus Christ know the truth? John told us in the previous verse that all of the believers had the anointing, the very Spirit of God living within them. This truth is very powerful for all of us to consider. The Spirit of God has many attributes and helps us in many ways. One of the most important things that he does for us is teach us the truth! The Apostle John recorded Jesus' words concerning the Holy Spirit in his Gospel:
 But the Helper, the Holy Spirit, whom the Father will send in my name, he will teach you all things and bring to your remembrance all that I have said to you. (ESV)
The Holy Spirit is our teacher. Many false teachers can come and go, but the perfect teacher of righteousness lives within us and reminds us of what Jesus has said. Now, it is critical to know that the Holy Spirit teaches that which the Savior has already said. He will never contradict the written Word of God, the Bible. Those who claim that the Holy Spirit has taught them knew teachings that are contrary to God's Word are very simply false teachers.
The Holy Spirit is your teacher. Are you a good student? What are you doing with your knowledge? Are you sharing it with others? Ask the Holy Spirit to not only teach you but to help you to be a good steward of what you have learned.