Luke 24:49

And behold, I send forth the promise of my Father upon you: but tarry ye in the city, until ye be clothed with power from on high.

This verse also, as indicated by Luke 24:48, was addressed to the apostles. They were here instructed not to begin the task of worldwide evangelism until they had been clothed with power from on high. Jesus also told them that "Ye shall receive power, when the Holy Spirit is come upon you" (Acts 1:8). The power was to come after the Holy Spirit came upon them; and, since this event occurred upon the first Pentecost after the resurrection, it is quite correct to identify that Pentecost as the beginning of the gospel age, the birthday of the church, the beginning of Christ's reign upon the throne of David, etc. All of this is clearly evident in Acts 2.