It is a miracle and a personal sign. It is the consistent sign that one has received the Holy Spirit.
A better way to phrase this probably "Who" is the Holy Spirit?
When we are considering God, why did He create us, or when we need Divine Help, or what is Eternal Life, or is Heaven real? We are dealing with a dimension we do not know much about. That is why there are so many religions, each giving their interpretations.
You may have heard some commonly used "answers" before like - "look at Nature or Creation..how can there Not be a Creator". God indeed has dazzled us with the wonders of His Creation and the Universe. These answers are correct but they may not completely satisfy you.
It is God's written Manual to us.
God uses the Bible to communicate to us His purpose for creating us, what He intends for us and how we can come to know Him personally and intimately.
The Christianity Jesus came to prepare man to have is a real and personal relationship with God. The term Christian actually means a person who is "anointed" by Christ. This anointing means that the believer is "set apart", and made "holy" by Jesus Christ with the Holy Spirit.