He makes us realize, believe, and testify of Christ.
The Holy Spirit, whom Christ will send, reminds and convicts us of the words of Christ, that Jesus is the Christ. (John 14:26, John 15:26)
Jn 14:26 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.
Jn 15:26 But when the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceedeth from the Father, he shall testify of me:
In fact, the disciples followed Jesus for three and a half years and listened to what Jesus was saying. But the disciples did not realize that Jesus was the Christ until He was resurrected. It was only after Jesus was resurrected that the Old Testament and the words of Jesus were understood. In other words, it was realized that Jesus was the very Christ. (John 2:22)
Jn 2:22 When therefore he was risen from the dead, his disciples remembered that he had said this unto them; and they believed the scripture, and the word which Jesus had said.
When the Holy Spirit comes upon us, we will spread the gospel to the whole world that Jesus is the Christ. (Acts 1:8)
Acts 1:8 But ye shall receive power, after that the Holy Ghost is come upon you: and ye shall be witnesses unto me both in Jerusalem, and in all Judaea, and in Samaria, and unto the uttermost part of the earth.
In the Old Testament, it is promised that the Holy Spirit will come upon those who obey God. (Joel 2:28)
Joel 2:28 And it shall come to pass afterward, that I will pour out my spirit upon all flesh; and your sons and your daughters shall prophesy, your old men shall dream dreams, your young men shall see visions:
The Holy Spirit did not come on the Jews who kept the law, but on those who believed in Jesus as the Christ. In other words, to believe in Jesus as the Christ is to obey God. (Acts 5:30-32)
Acts 5:30 The God of our fathers raised up Jesus, whom ye slew and hanged on a tree. 31 Him hath God exalted with his right hand to be a Prince and a Saviour, for to give repentance to Israel, and forgiveness of sins. 32 And we are his witnesses of these things; and so is also the Holy Ghost, whom God hath given to them that obey him. When the Holy Spirit comes upon us, we teach and preach that Jesus is the Christ wherever we go. (Acts 5:42)
Acts 5:42 And daily in the temple, and in every house, they ceased not to teach and preach Jesus Christ.