Its meaning was hidden from them

Luke 18:31-34
31 Jesus 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. 32 He will be delivered over to the Gentiles. They will mock him, insult him and spit on him; 33 they will flog him and kill him. On the third day he will rise again.”
34 The disciples did not understand any of this. Its meaning was hidden from them, and they did not know what he was talking about.

Yshua had explained to this disciples the things that must happen to Him, because that was God's mission and will for Him. It is not because He used complicated words that the disciples did not understand. They most likely not understand because Yshua was describing His own defeat, rather than victory that Messiah will bring. Yshua's defeat in man's eyes, is victory in God's eyes because by His death on the cross and the Resurrection, He defeated death and saved His people. We cannot understand God if we see things through our own eyes, instead we must simply trust Him that He is Lord and is in full control. It is no wonder He said childlike unquestioning faith is necessary to come to Him.

Jesus took the Twelve aside

Luke 18:31
Jesus took the Twelve aside and told them ...

Jesus had chosen twelve men in His ministry. These twelve will be imparted with his teachings and power to spread the news of the Kingdom. We are inclined to think that there are 11 good men and 1 bad man who betrayed our Lord. However, to Yshua, all have fallen short of God's glory. Although we know Judas fell, perhaps others may have fell or chosen to follow their own wills too. God did not choose the men because they were good, but rather all twelve of them were willing to follow Him. People cannot become Christians because they have some useful abilities, rather they know their moral poverty - poor in spirit - hence willing to be led by Yshua Ha Masiach.

Everything that is written by the prophets about the Son of Man will be fulfilled

Luke 18:31
Jesus 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. 

Our Lord Yshua had a very focussed mission. From start to finish he knew what He was going to do. However, He always had time for other requests that came up throughout His ministry. Nevertheless, as He reminded His disciples, His mission had already been written by the prophets hundreds of years before. For us today, even though we have a will to choose, God has plans to accomplish His will. We can surrender our wills, just as we surrender ourselves to Lord Yshua and be part of God's will, or we can follow our own course totally separate to God's will. It may be worthwhile noting that those close to Him, will be compelled to follow Him by the Holy Spirit.

And as they followed they were afraid

Mark 10:32
Now they were on the road, going up to Jerusalem, and Jesus was going before them; and they were amazed. And as they followed they were afraid. Then He took the twelve aside again and began to tell them the things that would happen to Him:

As Jesus is approaching Jerusalem, the disciples were uneasy, even afraid as described above. By the Holy Spirit the disciples knew who Jesus is supposed to be but we know from the despair following the crucifixion, that they still did not fully comprehend His Mission at this time.

So the disciples sense imminent danger as they approached Jerusalem with their Master. They have walked, talked, eat, slept with Jesus for over three years. But now, their Master's intentions remains mysterious and scary.

On various occasions, we have seen that God does not intend to be mysterious to His people, especially those close to Him. He will reveal His plans to us. Although we may walk closely with Him, His purpose are still so much greater than ours. It is no wonder that after years of following Him, we may still find ourselves walking far behind. However, we know that He is our refuge and fortress, we need only to trust and obey.

Do you also want to go away?

 John 6:67
Then Jesus said to the twelve, “Do you also want to go away?”

Jesus said the above following a somewhat difficult teaching for understanding as well as following. Jesus was teaching that we must eat His flesh and drink His blood to get eternal life. He is the ultimate manna that was provided by God to the people. For various reasons, various people cannot accept this or were offended by the sayings.

At this point many of those who were disciples (not the twelve), chose to turn away and Jesus understood this. This is where the true believers were differentiated. Jesus also explained that it is God who gave people understanding and faith to follow Him, and not because people chose based on their own understanding. So Jesus said the above question to his twelve inner disciples, and Peter moved by God, declared that Jesus is the Son of the living God.

The twelve asked Him about the parable

Mark 4:10
But when He was alone, those around Him with the twelve asked Him about the parable.

Continuing from the previous note, we again see that Jesus explained the parables to His disciples. Teaching in parables seems to be the normal way Jesus taught the crowd. However, as with most of the crowds, His disciples also did not understand. This account shows that the disciples did not hesitate to ask Jesus, they were willing to learn and did not have pride that prevented them from asking. 

Jesus spoke in parables for several reasons, firstly, His message is only intended for those He has chosen, secondly, His teachings are very confrontational to the religious views in the society, so He was being discreet at the appropriate time. 

Relating to us now, the Scripture may seem mysterious to us in certain parts, whereas other parts are very clear. God chooses to reveal the right things to us at His right time. We can just trust Him to show us. In addition, we can also be like the disciples and ask God for explanations and it would be His Spirit that leads us to ask the right questions at the right time. 

The great city, the holy Jerusalem, descending out of heaven from God, having the glory of God

Revelation 21:9-21

The same angels that had the seven bowls filled with seven plagues, explained to John about the Bride of Christ. In other context the Bride is understood to be the Church. However, in this context, the Bride is the new Jerusalem. The new holy Jerusalem descends from heaven. It has twelve gates guarded by twelve angels. Each gate has the name of one of the twelve tribes of Israel. Similarly there are twelve foundations of the city and each has the name of one of the twelve apostles of Y'shua.

A very detailed description of the New Jerusalem is given. The dimension of the square city is twelve thousand furlongs on each side. The walls are 144 cubits. Notice that the dimensions are twelve related. The city is pure gold. The foundations of the walls of the city has many precious stones, like jasper, sapphire, chalcedony, emerald, sardonyx, sardius, chrysolite, beryl, topaz, chrysoprase, jacinth, amethyst. The twelve gates were twelve pearls, hence the expression "pearly gates".