I really always think of Jesus as being confident, especially when he is speaking to his disciples. However, I never really associated it with what Barclay speaks of here ... the seemingly insurmountable odds against success.
One thing leaps out of this passage -- the confidence of Jesus. He has just been speaking of his death; he has no doubt that the Cross stands ahead of him; but nonetheless he is absolutely sure that in the end there will be triumph...
The last part of the passage has caused much serious thought. Jesus says that many who are standing there will not die until they see the Kingdom coming with power. What worries some people is that they take this as a reference to the Second Coming; but if it is, Jesus was mistaken, because he did not return in power and glory in the lifetime of those who were there.
But this is not a reference to the Second Coming at all. Consider the situation. At the moment Jesus had only once been outside Palestine, and on that occasion he was just over the border in Tyre and Sidon. Only a very few men in a very small country had ever heard of him. Palestine was only about 120 miles from north to south and about 40 miles from east to west; her total population was 4,000,000 or thereby. To speak in terms of world conquest when he had scarcely ever been outside such a small country was strange. To make matters worse, even in that small country, he had so provoked the enmity of the orthodox leaders and of those in whose hands lay power, that it was quite certain that he could hope for nothing other than death as a heretic and an outlaw. In face of a situation like that there must have been many who felt despairingly that Christianity had no possible future, that in a short time it would be wiped out completely and eliminated from the world. Humanly speaking, these pessimists were right.
Now consider what did happen. Scarcely more than thirty years later, Christianity had swept through Asia Minor; Antioch had become a great Christian church. It had penetrated to Egypt; the Christians were strong in Alexandria. It had crossed the sea and come to Rome and swept through Greece. Christianity had spread like an unstoppable tide throughout the world. It was astonishingly true that in the lifetime of many there, against all expectations, Christianity had come with power. So far from being mistaken, Jesus was absolutely right.
The amazing thing is that Jesus never knew despair. In face of the dullness of the minds of men, in the face of the opposition, in face of crucifixion and of death, he never doubted his final triumph -- because he never doubted God. He was always certain that what is impossible with man is completely possible with him.
The Gospel of Mark
(The Daily Bible Series, rev. ed.)