Acts 1:4

And being assembled together with them, he charged them not to depart from Jerusalem, but to wait for the promise of the Father, which, said he, ye heard from me.

Assembled together ... The Greek text here may be translated "eating with them," and thus there were possibly many occasions when Jesus ate food with his apostles after he was raised from the dead. Luke also in his gospel mentioned Jesus' eating with the apostles (Luke 24:43); and Peter referred to it in Acts 10:41. To be sure, the Lord needed to do no such thing, but it was important for the apostles to witness such a thing.

Not to depart from Jerusalem ... Not until after Pentecost and the baptism of the Holy Spirit would the apostles become fully qualified preachers of the gospel, hence the command the Lord gave that they should remain in Jerusalem until they were empowered from on high by their reception of the Holy Spirit.

Wait for the promise of the Father ... This has reference to a definite promise of God delivered to the apostles by Jesus himself ("which, said he, ye heard from me"), corroborating exactly all that John recorded in the five Paraclete passages of his gospel, and thus vanquishing the conceit that the synoptists knew nothing of such a promise.

Thus the apostles were to wait in Jerusalem because the promise of the Father was not yet given, and without it they were without power to accomplish their divine mission. Also, the prophet Isaiah had written:

Let us go up to the mountain of Jehovah, to the house of the God of Jacob; and he will teach us of his ways, and we will walk in his paths: for out of Zion shall go forth the law, and the word of Jehovah from Jerusalem (Isaiah 2:3).

Thus it was foreordained of God that the gospel should begin in Jerusalem; and it is hard to imagine a more significant verse in the whole Bible. Religions which were launched from Boston, Rome, Salt Lake City, or anywhere else on the face of the earth except "from Jerusalem" cannot be identified with the "word of the Lord"!