The triune nature of God made it possible for Jesus to leave Earth and yet remain present with his followers. Jesus described his connection with his followers this way:
Yet a little while and the world will see me no more, but you will see me. Because I live, you also will live. In that day you will know that I am in my Father, and you in me, and I in you. Whoever has my commandments and keeps them, he it is who loves me. And he who loves me will be loved by my Father, and I will love him and manifest myself to him. (John 14:19-21, ESV)
The Greek word translated manifest in John 14:21, emphanizo (em-fan-id´-zo) means “to exihibit (in person) or disclose (by words)” (G1718). Emphanizo is also translated as appear and show. The root word of emphanizo is emphanes (em-fan-ace´) which means to be “apparent in self” (G1717). What Jesus likely meant when he said he would manifest himself was that he would be seen in the behavior of the believer that is living according to his commandments. In other words, believers that act like him are making it seem as if Jesus is still living with us in this world.
Jesus told his disciples that his absence would not prevent them from continuing his work. In fact, Jesus promised them they would be able to do even more than they had before. He said, “Verily, verily, I say unto you, He that believeth on me, the works that I do shall he do also; and greater works than these shall he do; because I go unto my Father. And whatsoever ye shall ask in my name, that will I do, that the Father may be glorified in the Son. If ye ask any thing in my name, I will do it” (John 14:12). The work Jesus was referring to was probably the spreading of the gospel. During his three-year ministry, Jesus visited much of the territory that was promised to Abraham and his descendants, but the goal of his ministry was to spread the gospel throughout the entire world (Matthew 28:19). Even today, some 2000 years later, there are still people that do not have the Bible available to them in their native language.
Jesus indicated another person was going to come and help his disciples achieve their mission. He said, “If ye love me, keep my commandments. And I will pray the Father, and he shall give you another Comforter, that he may abide with you for ever; even the Spirit of truth; whom the world cannot receive, because it seeth him not, neither knoweth him: but ye know him; for he dwelleth with you, and shall be in you” (John 14:15-17). The indwelling of the Holy Spirit was a completely new and different way of connecting with God. Before Jesus died, the Holy Spirit was not living in the world. The possibility that God could live inside a person was a phenomenal breakthrough that Jesus’ disciples were most likely unable to comprehend.
Jesus briefly explained to his disciples how this new relationship was going to work. He told them, “These things I have spoken unto you, being yet present with you. But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you” (John 14:25-26). In a nut shell, what Jesus was telling his disciples was that they didn’t have to worry about forgetting the things he had taught them. Jesus’ identification of the Holy Spirit as the “Spirit of truth” (John 14:17) meant that he was the essence of God’s word being brought to life within the born again believer. With the exception of John, this fantastic revelation likely went over the heads of all of Jesus’ apostles. It probably wasn’t until after the day of Pentacost, when the Holy Spirit arrived on the scene (Acts 2:1-4), that the reality of Jesus’ promise actually set in.