They did not know what He was talking about.  Why was the meaning of His words hidden from them?  He was telling them point blank what would happen and they just did not get it.

Luke 18:
31Jesus took the Twelve aside and told them, "We are going up to Jerusalem, and everything that is written by the prophets about the Son of Man will be fulfilled. 32He will be handed over to the Gentiles. They will mock him, insult him, spit on him, flog him and kill him.33On the third day he will rise again."
 34The disciples did not understand any of this. Its meaning was hidden from them, and they did not know what he was talking about.

There are times when repeating the truth seems to fall on deaf ears.  Something deeper is required for one to be persuaded.  The disciples heard it over and over.  They were not convinced until they saw Jesus after the resurrection.

They were so convinced after it came to pass, that they gave their lives to preach His death and resurrection.  Is it likely that they would go to the ends of the earth with Jesus’ message of Resurrection if they were not persuaded?

Everyone in the world is trying to persuade others of their rightness.  Are you any different?  Do you not have agendas and views that you want others to validate? 

The Resurrection of Jesus Christ from the dead is often questioned and rejected by people who have not opened their mind to the possibility of God working in ways beyond their understanding.  In short, words do not convince them to believe it. 

It is only because of God’s Spirit coming into us that we can believe.  The disciples believed when they saw their friend come back from the dead.  We believe when the same Spirit that raised Jesus from the dead empowers our faith.  We may not have seen the historical Jesus in the flesh, but God’s Spirit persuades us to believe what others were completely convinced of.  The Resurrection made believers out of half-hearted followers.