Today's Maundy Thursday (you know, the day before Good Friday, and the day on which Jesus ate the last supper with His disciples). Maundy Thursday's isn't really famous for what happened during the day, but rather for what happened that night. It was the night on which Jesus was betrayed. It was the night on which Jesus was arrested and tried before the High Priest. And before that it was the night on which He washed the disciples' feet and the night on which He prayed in Gethsemane. The Synoptic Gospels (Matthew, Mark and Luke) tell us a lot about what happened that night. John, on the other hand, tells us a lot about what Jesus said that night.
It was the night before the Cross, and yet Jesus spent a lot of it teaching His disciples. In fact, one of the largest chunks of Jesus' teaching to be found in the gospels, the Upper Room Discourse and High Priestly Prayer of John 13-17 was what He taught on that eventful night. Jesus knew what was coming. He had already been warning His disciples. And yet, He chose to spend the last night of His pre-Cross earthly ministry teaching these things. So they must be important things. If they weren't important, they could have waited. And we know that Jesus did wait to teach them some things after His resurrection, and yet other things were revealed to the writers of the New Testament Scriptures. So these things that Jesus taught on Maundy Thursday were very important indeed.
So, what were these important things about which Jesus spent His last night before being put to death talking? He didn't talk about the most effective evangelistic methods. He didn't talk about church leadership best practice. No, the really important things He talked about were the Lord's Supper, the Trinity and Union with Christ. I'm not going to go through all the Scriptures about that night, but just let me touch briefly on those three important topics of which Jesus spoke.