Our Scripture reading today is 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 power of the Holy Ghost would be given to the disciples for the purpose of witnessing. God's Spirit filling them would naturally lead to them being witnesses. This same filling and power are available today and will yield the same result: witnesses. If we are filled with the Spirit, we will have power. If we are filled with the Spirit, we will witness. We can diagnose whether or not we are filled with the Spirit by whether or not we are witnessing. A Spirit filled follower of Jesus Christ WILL be a witness. Matthew 4:19, "And he saith unto them, Follow me, and I will make you fishers of men."