v.24 Let that therefore abide in you, which ye have heard from the beginning. If that which ye have heard from the beginning shall remain in you, ye also shall continue in the Son, and in the Father.
In the context of this verse, the truth has to do with who Jesus Christ was and claimed to be. The Gnostics were already gaining a foothold in the church with a false teaching that Jesus did not really come in the literal flesh, but was actually a phantom. This teaching was propagated best by one named Arius and it eventually became known as the Arian Heresy. He eventually came to the conclusion that Jesus was not God, but a creation of God just like us. Sadly, this same teaching is the foundation of the Church of Jesus Christ of Latter Day Saints (Mormons) and Jehovah's Witnesses today. The Mormons say that Jesus is actually the brother of Lucifer (the Devil) and the Jehovah's Witnesses say that Jesus is actually Michael the Archangel. Either way, they both deny the deity of Jesus Christ and have left the truth.
Sadly, so many do the same thing today in the church. They come in long enough to hear the truth and then get lured away into error. They start out well, but then they take a detour. It is a big deal who you believe Jesus is. Was He just a great man or God Incarnate? It's no small issue. As a matter of fact, it is the none other than the difference between Heaven and Hell. Jesus said in John 8:24, I said therefore unto you, that ye shall die in your sins: for if ye believe not that I am he, ye shall die in your sins. In this verse, Jesus is invoking the name of God, "I am" (the word "he" is not in the original text). He is claiming to be the same God that said to Moses in Exodus 3:14 after he asked him His name, I AM THAT I AM: and he said, Thus shalt thou say unto the children of Israel, I AM hath sent me unto you.
The choice is ours to make. Jesus asks the same question of us today that He asked of Peter almost two thousand years ago in Luke 9:20, But whom say ye that I am? Josh McDowell says that each of us has to answer that question one way or the other. He is either Lord, Liar, or Lunatic to each of us. Who is He to you?