What does Matthew 26:59 mean?

The Jewish ruling body was known as the Sanhedrin. It included the high priest, along with 70 men made up of priests, teachers of the law, and laymen known as elders. It's likely not all 70 were gathered at this hasty late-night trial conducted in Caiaphas' mansion. Only 23 would have been needed to make any decisions official.

Matthew makes it clear this is only a "trial" by appearances. The chief priests and the council had already decided the verdict and the sentence: they wanted to put Jesus to death. They just needed evidence they can use to justify their prejudice. This leads them to seek false testimony about Jesus that would allow them to condemn Him. This is not only unethical, but punishable, in this case by death, under Mosaic law (Deuteronomy 19:18).

These ruling religious leaders hated Jesus for what He had said about them (Matthew 23:1–7). They rejected and despised what He said about Himself (John 15:18). They did not want to disrupt the status quo with the Romans (John 11:48), to the point of rejecting their own long-promised Messiah (John 5:39–40). Given His popularity (Mark 12:12) and invincibility in public debate (Luke 20:40), Jesus had to die if they were going to continue in their positions of power and influence over the people.