what Jesus said is because he knew that many of his disciple still have not understand he is the son of man as well as the son of god.

Jesus was saying this to them because of his purpose is to bring them to see what he said in the future is true, means Jesus knew the future of all and that is come true. this is Jesus proclaimed and told all his disciples that what he is saying is eternal truth and he does as what he has said to them.

Hence Jesus told them that this is the final penulty for sin or false that has been beyong with man.it is like what has created by god, god has purpose to restore or response.

 Final word of understanding that Jesus wanted to add in deeply was death is not finish yet, it is stil "life".