Through the power of the Holy Spirit we are able to bear witness of the Saviour.
This promise was made by our Lord to His disciples before His ascension: "And that repentance and remission of sins should be preached in his name among all nations, beginning at Jerusalem. And ye are witnesses of these things. And, behold, I send the promise of my Father upon you: but tarry ye in the city of Jerusalem, until ye be endued with power from on high" (Luke 24:47-49).
These words were fulfilled when the Holy Spirit was given on the Day of Pentecost. He began a work then that He now continues--indwelling believers and empowering them to witness as they give Him complete control of their lives.
This is confirmed by 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."
The disciples no longer hesitated to witness after the Day of Pentecost. We read in Acts 5:32: "And we are his witnesses of these things; and so is also the Holy Ghost, whom God hath given to them that obey him."
"And, behold, I send the promise of my Father upon you: but tarry ye in the city of Jerusalem, until ye be endued with power from on high" (Luke 24:49).