Jesus answered this question in John 14 with the disciples when he tells them “I am the truth”. Jesus can testify to the truth and teach the truth because he himself is that truth. In him, there is nothing false, nothing misleading, and nothing fake or uncertain.
Each of us are capable of knowing truth, but none of us can claim to actually be truth. There are too many things we don’t know, and too many things we get wrong throughout our lives. Yet Jesus claims to be truth, and in doing so claims to be one with God.