"These things I have spoken to you while being present with you. But the Helper, the Holy Spirit, whom the Father will send in My name, He will teach you all things, and bring to your remembrance all things that I said to you. Peace I leave with you. My peace I give to you; not as the world gives do I give to you. Let not your heart be troubled, neither let it be afraid. You have heard Me say to you, 'I am going away and coming back to you.' If you loved Me, you would rejoice because I said, 'I am going to the Father,' for My Father is greater than I. And now I have told you before it comes, that when it does come to pass, you may believe. I will no longer talk much with you, for the ruler of this world is coming, and he has nothing in Me. But that the world may know that I love the Father, and as the Father gave Me commandment, so I do. Arise, let us go from here."
Jesus declared that the Holy Spirit would teach the apostles all things and bring to their remembrance all that which He said to them. The work of the Holy Spirit among the apostles was exceedingly important, and this promise was exclusively given to them. They, as Jesus speaks to them, have little understanding of many things. But, there was no need to fear His departure because the Holy Spirit would teach them "all things" (i.e., everything God needed them to know and teach), which would include new revelations as well as bringing to remembrance certain truths they had heard Jesus speak but had forgotten or had not understood.
There is no promise to Christians today that the Holy Spirit will miraculously teach us or remind us of anything. In this age, God has revealed His will to us through His inspired word (cf. II Tim. 3:16,17). It is up to us to study it in order to know it, and we must regularly review the truths we have learned in order to prevent forgetfulness (cf. II Tim. 2:15; II Pet. 1:12-15).
Although they did not have peace at the moment, they would have an inner calm in the near future that no one could take from them. After Jesus' resurrection and ascension, and the Day of Pentecost, they would better understand the spiritual nature of the kingdom and their mission, and they would have the internal peace that accompanies such.
Disciples of today will also obtain this peace (even amidst the storms of life and persecution) when they learn to cast their burdens on the Lord, seek first His kingdom, and trust Him completely in all things (cf. I Peter 5:7; Matt. 6:33; Phil. 4:4-7). With the gift of "peace" from God, there is no need to be "troubled" or "afraid."
Although the apostles loved Jesus, their love was imperfect. Out of selfish reasons they did not want Jesus to depart from them, but they should have rejoiced in such for it meant that Jesus was returning to the Father and to glory (cf. Phil. 2:6-9).
"My Father is greater than I" - The Father is not greater than the Son regarding Their essence, for They are both deity and possess all the attributes of such. Perhaps Jesus is referring to the fact that in order to save man He has taken upon Himself a humiliating role (i.e., in becoming a man and dying in order to atone for the sins of the world). It could also be that Jesus is referring to the fact that the Father is greater than He is regarding authority (cf. John 14:24; I Cor. 11:3; 15:24-28).
"And now I have told you before it comes, that when it does come to pass, you may believe" - One of Jesus' purposes in this chapter is the encouragement of His apostles. The fulfillment of detailed, predictive prophecy would encourage them and strengthen their faith in the coming days.
"I will no longer talk much with you, for the ruler of this world is coming, and he has nothing in Me" - Their time together was indeed running short. In a few hours Satan, "the ruler of this world," would come (through his agents like Judas Iscariot) and arrest Jesus. The fact that Satan "has nothing" in Jesus simply means that he had no power or authority over Christ; he had no right to seize Jesus as he was about to. The devil can only do that which God permits him to do.
"But that the world may know that I love the Father, and as the Father gave Me commandment, so I do" - Jesus would submit to being arrested, tried, and wrongfully put to death, but only because He loved the Father (and mankind) and had been instructed to do so. The humiliation He would endure would show His love for the Father.