Jesus had not yet been arrested, but the trial of public opinion had already began. Jesus testified that He’s the light of the world. In court, both the testimony and the witness must be creditable. The witness affirms that they are telling the truth, yet experience has taught that many witnesses lie anyway. A witness that is well known for being honest and trustworthy is much more likely to be believed. If the witness is unknown, then the jury is much more likely to believe or disbelieve testimony based on how it fits their own beliefs.
The Pharisees were unrighteous, and they did not want to believe Jesus. They desired power, wealth and glory. If Jesus was right, and his testimony was true, then their way of life would be threatened. They had every motive to want to prove Jesus to be a fraud. To discredit Jesus, their first attack was upon the testimony of Jesus. If they could prove that His claim was a lie, then that would prove He was a liar. To do this, they took an Old Testament law and twisted it. They then applied it in an illegitimate way.
The law they used was a requirement to have two trustworthy witnesses to find guilt in a criminal trial. It did not require two witnesses for testimony to be true. Jesus was not giving testimony in a criminal case. He was testifying as to who he was and where He was from. His testimony was true.
Jesus accused them of judging by human standards, and therefore their judgments were corrupted. Jesus knew the truth about Himself; they did not. Jesus then told them that God was His second witness. Hearing God’s voice was difficult, but the Old Testament tells of many who had heard God’s voice and obeyed. The Pharisees claimed to serve God, but they did not hear God’s testimony about Jesus. Jesus made it easy for everyone. By knowing Him, we can know God His Father.