Nowhere in the bible does it say that holy spirit is god. But it is the invisible energizing force that God puts into action to accomplish his will. It is holy because it comes from God, who is clean and righteous, and because it is God's means to accomplish what is holy. One example is found at Luke 1:35- "in answer the angel said to her: "Holy spirit will come upon you, and power of the Most High will overshadow you. And for that reason the one who is born will be called holy, God's son." Another example is found at Acts 1:8- "but you will receive power when the holy spirit comes upon you, and you will be witnesses of me in Jerusalem, in all Judea and Samaria, and to the most distant part of the earth." These are just a few examples.