In the book of John chapter 14 Jesus promised that He would not leave His disciples alone, but would send them the Comforter, the Holy Spirit, who would be with them every day.
The first thing we should know is that God keeps His promises, and we see this great promise fulfilled in the book of Acts, but before it was fulfilled, Jesus reminded them that this was going to happen:
6 When they therefore were come together, they asked of him, saying, Lord, wilt thou at this time restore again the kingdom to Israel?
7 And he said unto them, It is not for you to know the times or the seasons, which the Father hath put in his own power.
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.