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.
Exhortation: Joan Boney ... apostle/prophet
A person can try to "witness" and be powerless unless the Holy Spirit is speaking through him.
An example: Many years ago, I was playing golf with two women. One woman was Church of Christ and the other woman Methodist. The Methodist woman began speaking about people judging other people. (I felt she was speaking about me.) Through me, by the Holy Spirit, came the following words: "If you are judging someone is judging aren't you judging?"
This woman was like a puffed up balloon that has been pricked with a pin. She actually gasped! The Holy Spirit took all the air out of her.
Another example: I was at a copy center when I little old man came up to me and said: "Little lady ... you didn't park behind that green and gold car did you?" I replied, "No sir." Then he said, "That's a Baylor car (Baylor Baptist University in Texas)" ... I said, "OK". Then he said, "You don't happen to be a Christian do you?" To this I was very excited for I had only been a Christian a few weeks when this happened. I said, "Well ... yes I am." A frown came on his face and he said, "Oh, heck ... I just joined the witnessing class at First Baptist Church and you are the first person I tried to witness to." He was disappointed because I am a Christian!
When we set about to "witness" we work in our own flesh.
When we are a witness due to what God has done in us, it will not be in vain.
There is a big difference between "trying" to witness and being a witness. One is of the flesh and profits nothing. The other is of the Spirit and does the work intended by God.