Our second question from John 8:31 is: What is Jesus referring to by the phrase, “my word”? “If you abide in my word, you are truly my disciples.”
The word is singular, “my word,” not “my words.” This means that Jesus is thinking of the sum of all that he has taught. We could leave it at that: Jesus means “abide in the sum of all that Jesus taught.” But my guess is that Jesus wants us to ponder what the sum of that word is. And surely the answer to that is: He is the sum of his word. All his words in one way or another draw our attention to him.
Words like: “I am the bread of life” (John 6:35). “I am the light of the world” (John 8:12). “I am not of this world” (John 8:23). “I am the good shepherd” (John 10:11). “I am in the Father” (John 10:38). “I am the resurrection and the life” (John 11:25). When you take all his words together, they have one great focus—Jesus himself. “These are written—all these words are written—so that you may believe that Jesus is the Christ, the Son of God” (John 20:31). They all point to him.
Which is why when you get to chapter 15, Jesus can say, not only “abide in my word,” but “abide in me.” “If anyone does not abide in me he is thrown away like a branch” (John 15:6).
So the answer to our second question would be: The phrase “my word” here in John 8:31 (“if you abide in my word”) refers to the sum of Jesus’ teaching which is summed up in himself and all that he is for us as the crucified and risen Son of God.
– from John Piper’s sermon If You Abide in My Word, You Are Truly My Disciples