1 John 5:20
 And we know that the Son of God has come and has given us understanding, so that we may know him who is true; and we are in him who is true, in his Son Jesus Christ. He is the true God and eternal life. (ESV)
John is coming to the very end of this letter written to a church that was traumatized by false teaching. The false teachers did their best to deny the truth of Jesus Christ as both fully human and fully God, being the only true sacrifice for the sins of mankind. The church had been infiltrated by falsehood. John reminds them of that which is true. Truth. Genuine truth. It is vital.
As we look at this verse we discover how truth is discovered. John begins this verse by saying that we know that the Son of God has come and has given us understanding to that we may know him who is true. Christ came and gave us understanding. Would we be able to discern the truth without the Lord's help? That's an emphatic NO! Jesus came and revealed the truth of who he is to us.
The false teachers were also known as "Gnostics." The word gnostics comes from the greek word for knowledge. These teachers claimed to have special or secret knowledge that only they had. Their special knowledge led them to a false understanding of who God is. It is so important that we believe in the Son of God, and in his revelation of who he is. All other sources of knowledge are false.
In the second part of this verse we are reminded of who we are. This church had their faith shaken. As a result of dealing with this false teaching they wondered who they were in relationship to God. John reminds them that they are in him who is true. They are in Jesus Christ. They are in relationship with God and have eternal life. What an encouragement.
Devote yourself to the truth. If someone claims to have special knowledge do not be impressed by them. Take their truth claims to the Word of God. Those who know the truth know God himself.