ANSWER: They needed to wait for the Holy Spirit.

Just before Jesus was lifted into heaven on a cloud before his disciples’ eyes, Jesus told His Apostles to stay in Jerusalem because the Holy Spirit was going to come to them. 

Sure enough, fifty days after the Resurrection (and just 10 days after Jesus gave the instructions), the Apostles and Mary were all together in a room when the Holy Spirit came down on each of them in the form of tongues of fire. It was a huge, transforming event in their lives, not to mention they instantly could speak all different languages.  This day is known as  Pentecost

BIBLICAL ACCOUNT OF JESUS TELLING THEM NOT TO EXIT JERUSALEM BEFORE THEY RECEIVED THE HOLY SPIRIT

While meeting with them, he enjoined them not to depart from Jerusalem, but to wait for "the promise of the Father ³ about which you have heard me speak;

for John baptized with water, but in a few days you will be baptized with the holy Spirit."

When they had gathered together they asked him, "Lord, are you at this time going ⁴ to restore the kingdom to Israel?"

⁵ He answered them, "It is not for you to know the times or seasons that the Father has established by his own authority.

⁶ But you will receive power when the holy Spirit comes upon you, and you will be my witnesses in Jerusalem, throughout Judea and Samaria, and to the ends of the earth."

When he had said this, as they were looking on, he was lifted up, and a cloud took him from their sight (Acts 1: 4-9)

Biblical Account of Pentecost (10 days after Jesus ascended into heaven):

And when the days of the Pentecost were accomplished, they were all together in one place:

And suddenly there came a sound from heaven, as of a mighty wind coming: and it filled the whole house where they were sitting.

And there appeared to them parted tongues, as it were of fire: and it sat upon every one of them.

And they were all filled with the Holy Ghost: and they began to speak with divers tongues, according as the Holy Ghost gave them to speak.

Now there were dwelling at Jerusalem, Jews, devout men, out of every nation under heaven.

And when this was noised abroad, the multitude came together, and were confounded in mind, because that every man heard them speak in his own tongue.

And they were all amazed, and wondered, saying: Behold, are not all these that speak Galilean?

And how have we heard, every man our own tongue wherein we were born?

Parthians and Medes and Elamites and inhabitants of Mesopotamia, Judea, and Cappadocia, Pontus and Asia,

Phrygia and Pamphylia, Egypt and the parts of Libya about Cyrene, and strangers of Rome,

Jews also, and proselytes, Cretes, and Arabians: we have heard them speak in our own tongues the wonderful works of God.

And they were all astonished, and wondered, saying one to another: What meaneth this?”

[Acts 2:1-12]

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