What does John 12:48 mean?

Earlier, the gospel of John pointed out that the purpose of Jesus' first coming—His birth, ministry, and death—was to establish a means of salvation for all who believe in Him (John 3:16–17). The same gospel, however, also makes it clear that those who reject Christ will have no part in that salvation (John 3:18, 3:36). Jesus Christ, God the Son, is identical to God the Father (Colossians 1:15; Hebrews 1:3). A person who rejects Jesus rejects God (John 8:19; 2 Corinthians 4:4).

The ultimate return of Jesus will not be a mission of mercy, forgiveness, or humility. His second coming will be for judgment on sin, not to be a living sacrifice (Hebrews 9:27–28). These words are part of Jesus' final public teachings prior to His arrest and eventual execution.

Part of the judgment which will be used against those who deny Christ is the ample evidence they have been given. In that sense, "the word that [Jesus has] spoken" becomes evidence against them. This applies to more than just people who saw Jesus in person. It includes every single person who has ever lived, because God has made Himself plain enough so that no one has an excuse for disbelief (Romans 1:18–20). Some work harder than others to deny the truth (John 5:39–40). However a person comes to unbelief, the end result will be the same: those who have not believed in Christ will be eternally lost (Revelation 19:11–16; 20:11–15).