We will receive power, when the Holy Ghost comes upon us  -  Acts 1:8

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.

Before Christ ascended into heaven, to be restored to His position on the right-hand of the Father, He directed the disciples’ attention to the nature and sphere of their mission. As to its nature – they were to be witnesses, as to its sphere – they were to be witnesses in Jerusalem, Judea, Samaria, and all the earth. But first they must receive the power of the Holy Spirit. Jesus knew that without the Holy Spirit’s power, they would be ineffective. The disciples needed power for witnessing and holy boldness for preaching the gospel. They could only receive that power, when the Holy Spirit came upon them.