The unpredictability of Jesus’ itinerary led the disciples to accept a day-by-day lifestyle.  They did not seem to have a long-term perspective, except perhaps hoping that Jesus would become King of Israel … and that they might have important roles in His ‘government’ (Mark 9:33-37).  So Matthew records three reality-checks in his gospel, in which Jesus plainly tells the disciples that He would die at the hands of violent men.
 
The first (Matthew 16:21) was after Peter's declaration that Jesus was the Christ (Messiah), when the disciple rebuked Jesus for even thinking He might be killed - Jesus said strongly, "Get behind me Satan …" (Matthew 16:23).  The reality-check in today's verses (Matthew 17:22-23) came after the transfiguration.  The third (Matthew 20:17-19), came just before James’ and John’s mother asked Jesus for her sons to have the most senior jobs when Jesus became King of Israel (Matthew 20:20-28).  But Jesus would not allow fantasy, wishful-thinking or proud ambitions to interrupt His salvation plan.
 
The disciples still did not understand that it was necessary for Jesus to die (Luke 24:25-27) because they did not understand the Scriptures (John 20:9).  His words only brought grief.  Even though Jesus promised resurrection, they only understood death.  They would not allow any room for the idea of Jesus being killed: surely it would not only be the end of Him but also of them.   They had no idea that Christ's resurrection would be the foundation of their future.  That would become the gospel which they would take to the world, and upon which the church (including us) would be built.
 
God does not bring grief to the believer except to discipline and strengthen us.  Hebrews 12:5-6 says, "My son, do not make light of the Lord's discipline, and do not lose heart when he rebukes you, because the Lord disciplines the one he loves, and he chastens everyone he accepts as his son."  Lamentations 3:31-33 says, "For no one is cast off by the Lord for ever. Though he brings grief, he will show compassion, so great is his unfailing love. For he does not willingly bring affliction or grief to anyone."  God's reality checks are not to take away hope but to establish hope on the right basis.  So, do not be discouraged: allow God’s reality to displace your fantasy. Then you can be confident of His saving love, and that He will provide for your future.