That which was from the beginning, which we have heard, which we have seen with our own eyes, which we have gazed upon and touched with our own hands—this is the Word of life. 

And this is the life that was revealed; we have seen it and testified to it, and we proclaim to you the eternal life that was with the Father and was revealed to us.

We proclaim to you what we have seen and heard, so that you also may have fellowship with us. 

And this fellowship of ours is with the Father and with His Son, Jesus Christ. 

We write these things so that our joy may be complete. 1 John 1:1-4

John clearly affirms all things he is writing about Jesus is his firsthand eyewitness account. 

If you think about it he wasn't gaining anything by proclaiming the good news of Jesus the Messiah. In fact he was killed for spreading the gospel. What do you think compelled him and many others to do that, if not the fact that it was the truth and they had witnessed it in person?

If you still doubt, I strongly suggest that you ask Jesus to reveal Himself to you and He will. 

You will seek me and find me when you seek me with all your heart. Jeremiah 29:13