The Witness
Imagine that you are a defendant in a courtroom, and you are called to the stand to give your side of the story. No matter who you are or how well spoken you are, it would be foolish for the jury to just take you at your word without anyone else corroborating your story. That is why witnesses are so important in a courtroom.
In Acts 5, Peter invokes the Holy Spirit himself as a third witness to all that Jesus had done. In that text, as well as several others, the Holy Spirit is described as a "witness." And as we find out by digging into the scriptures, the Holy Spirit's role as a perfect witness to the gospel and to what is true is extremely vital to us and our faith, especially considering how untrustworthy our own emotions and thoughts can be.