Jesus told them some things that the Holy Spirit would do.

John 16:5-15

5 “Now I am about to return to the one who sent me. But I am disappointed that none of you is asking me, ‘Where are you going?’ 6 Instead, because I have told you these things, you are very sad. 7 But the truth is that it is good for you that I am going away, because if I do not go away, the Holy Spirit, who will ◄encourage/be like a legal counsel for► you will not come to you. But when I go, I will send him to you. 8 When he comes, he will prove that those who do not belong to God [MTY] are wrong about what is sinful and about who is really righteous and about whom God will judge and condemn for their sin. 9 He will tell people that their greatest sin is that they do not believe in me. 10 He will tell people that because I am going back to my Father, and you will no longer see me, you will know that I am the one who was truly righteous. 11 He will tell people that the fact that God has already determined that he will punish Satan, the one who rules this world, shows that some day God will also punish those who do not belong to him.

12 I have many more things that I would like to tell you, but you are not able to accept them now. 13 But God’s Spirit is the one who will teach you God’s truth. When he comes, he will guide you so that you understand all spiritual truth. He will not speak from his own authority. Instead, it is the things that he hears my Father say that he will tell you. He will also tell you about things that will happen later. 14 He will honor me by revealing my truth to you. 15 Everything that my Father has is mine. That is why I said that the Spirit is able to reveal my truth to you.”