Peter, James and John were confused.  They had just seen Jesus transfigured along with Moses and Elijah.  They had heard Father God affirming Jesus as His Son, and commanding them to accept the authority of His words (Matthew 17:1-5).  It seemed an amazing prelude to Jesus’ glorious reign.  But then, Jesus said they must not tell anyone until He had risen from the dead (Matthew 17:9).  What did that mean?  

Also, the religious scholars said that Elijah had to come before the Messiah as Malachi had prophesied (Malachi 4:5-6).  Then, at the transfiguration, Elijah appeared and was quite recognisable alongside Moses.  Was this what the Rabbis were talking about?  Jesus affirmed Malachi's prophecy.  In fact, it had already been fulfilled by John the Baptist (Isaiah 40:3; Luke 1:13-17; Matthew 3:3).  Although Jesus had previously explained that in Matthew 11:11-14, the disciples had not understood.  

So Jesus told them again that what happened to John the Baptist would happen to Him.  John was arrested and died in an unjust execution: so too would Jesus.  He was now preparing His disciples to be prepared for the time when He would suffer.  After that, they would lead the church through difficult periods of persecution (including the violent death that many of them would face).  It became a pattern for all those who witnessed their faith in Jesus (the Greek word for ‘witness’ is ‘martys’ - from which we get ‘martyr’ in English).  
 
Nothing happens 'by accident'.  The Lord does not show us all of our life ahead, but He lets us see as far as is necessary for us to cooperate with Him.  He is going to fulfil His plan and wants to get us ready to be used, in one way or another, to be involved so that He will achieve it.  John had been a willing participant and so was Jesus, and the apostles after Him.  Will you allow the Lord to use you in whichever way He shall choose?