Jesus' first question is an open challenge: can anyone here prove Me guilty of sin? Barnes explains that "convince" here does not mean persuade: "It rather means to convict. Which of you can prove that I am guilty of sin?" Gill says that whatever they had charged Him with, "none of them all could prove anything against him, so as to convict him according to law."

The second question follows from the first. If no fault can be found in the speaker, then the problem with their unbelief lies somewhere else. Barnes sums up the argument: "A doctrine might be rejected if it could be proved that he that delivered it was an impostor; but as you cannot prove this of me, you are bound to receive my words."

No one named a single sin. Jesus answered His second question Himself: "He that is of God heareth God's words: ye therefore hear them not, because ye are not of God" (John 8:47). Instead of answering His challenge, the Jews turned to insults: "Say we not well that thou art a Samaritan, and hast a devil?" (John 8:48).

In older English, "convince" meant to prove someone guilty, and the ESV says "convicts." The Greek verb is elencho, which Thayer defines as "to convict, refute, confute, generally with a suggestion of the shame of the person convicted." John uses the same verb when he says evildoers avoid the light "lest his deeds should be reproved" (John 3:20), and when Jesus says the Spirit "will reprove the world of sin" (John 16:8).

Gill notes that His enemies did bring charges against Him, such as calling Him a glutton and a winebibber (Matthew 11:19), and charging Him with blasphemy, but none of these could be proved.

Jesus asked this question of people who wanted Him dead (John 8:40), and no one could answer. Gill draws the conclusion: since no sin in life and no corruption in doctrine could be proved against Him, "what he said must be truth." His life stands behind His words: the One no one could convict of sin also speaks the truth He has "heard of God" (John 8:40, 45).

That moves the question from the messenger to the hearers. If He tells the truth, unbelief has no ground in Him. Jesus' own answer points to the heart: hearing God's words is the mark of being "of God" (John 8:47).