"Then he said unto them, O fools and slow of heart to believe all that the prophets have spoken. Ought not Christ to have suffered these things, and to enter into his glory?"
Those words were spoken by Christ on the day of His resurrection: spoken not to unbelievers but to Believers. The disciples to whom He was speaking were not balanced in their theology: they believed a certain part of God's truth and they refused to believe another part of the truth that did not suit their own hopes and will; they believed some of the Scriptures but they did not believe all of the Scriptures that the prophets had spoken, and the reason they did not was because they were unable to harmonize their will and God's truth. They were like some people today: when it comes to their theology; they walk by reason and by logic of their own understanding rather than by faith and the Word of God.
In the Old Testament there were many prophecies concerning the coming Messiah that spoke of His glory. If there was one thing the Old Testament predictions made plain, it was that the Messiah of Israel would be GLORIOUS. It spoke of His power, His honor, His majesty, His dominion, His triumphs. But on the other hand, there were many prophecies in the Old Testament that spoke of a suffering Messiah, that portrayed His humiliation, His degradation, His rejection, His death at the hands of wicked men. And these disciples of Christ believed the first set of prophecies, which was to their liking, but they would not believe in the second set. They thought that Christ would save the Jews in their day and they would reign with Him in His earthly Kingdom at that time. The apostles could not see how it was possible to harmonize the two sets of prophecies. If the coming Messiah was to be a glorious Messiah, possessing power and majesty and dominion: if He would be triumphant, then how could He, at the same time, be a suffering Messiah, despised, humiliated, rejected of men, and killed on the cross with thieves? And because the disciples could not reason the two together, because they were unable to harmonize them, they refused to believe, then Christ told them to their faces that they were fools. He says, "O fools, and slow of heart to believe ALL that the prophets have spoken."
I suppose some of us have wondered how it was possible for these disciples, these followers of Christ, who had been privileged to be with Him during His public ministry and those who had been so intimate with Him, had been instructed by Him, had witnessed His wonderful miracles; how was it possible for such men to err so grievously and to act so foolishly?
And yet we need not be surprised; the same thing is happening all around us even today. Our world is full of denominations and ministries run by men and women who believe and teach only portions of God's truth and even add or subtly inject their own ideas as gospel. They do not believe all that the prophets have spoken and at the same time put words in their mouths. For example; many say that Christ abolished the law and it is no longer in effect when the prophets of old, who foretold all of Christ’s ministry in great detail, never made such a profound prophesy. The prophets and Christ, on the other hand, say exactly the opposite;
But this shall be the covenant that I will make with the house of Israel; After those days, saith the LORD, I will put my law in their inward parts, and write it in their hearts; and will be their God, and they shall be my people. Jer 31:33
Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfil. For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled. Matt 5:17-18
Now notice that Christ did not upbraid His disciples because they did not understand, but because of their lack of belief. They prided themselves on their logical minds and desires, rationalizing that the Messiah of Israel is going to be a glorious and a triumphant Messiah, or else He is going to be a rejected and a humiliated one: they cannot both be true. That is why Christ called them fools, not because of their failure to understand, but because of their lack of faith and then replied: "Ought not Christ to have suffered these things, and to enter into his glory?".
I am afraid that today there are many who only believe what they would like to believe, and if there is something else that they cannot understand or is not what they were led to believe, they reject it without further study. If they have devised to themselves a systematized theology (or more probably they have adopted someone else's system of theology), and they hear a sermon (no matter how much Scripture there maybe in it) which they cannot fit into their little system of theology, they won't have it. They place a higher value on consistency and tradition than they do on fidelity and Scripture. It was the same matter with the disciples: they could not see the full picture of Christ's mission and therefore they were only prepared to believe what they wanted to see instead of the truth.