Luke 18:34 (ESV)
34 But they understood none of these things. This saying was hidden from them, and they did not grasp what was said.
First, I am comforted that the men who God used to turn the world upside down had moments when they just did not get it. They, at times, were just as obtuse and blind as I often am. I feel as though I am in good company and take encouragement from it.
My second thought is concerning the veracity of the Gospel accounts. One of the lines of argument of atheists is that the New Testament was compiled by church leaders in later centuries to support the claims of the faith. In other words, the leaders made up stories to support their theological positions.
If I were making up stories about the men from whom I inherited my authority, I don’t think that I would include any statements that would denigrate those men.
Why would Luke include this statement in his Gospel? The only logical answer is because it was true and shows that the spread of the Gospel is not due to the incredible wisdom of the Apostles. They were regular men, just like you and I. They had the same periods of stupidity and struggled in their faith like we do.