In short
God the Holy Spirit
God the eternal Spirit worked with the Father and the Son in creation, incarnation and redemption. He inspired the writers of Scripture, fills Christ's followers with power, draws people to Jesus and leads those who respond into the family of God.
Read it in the Bible
Genesis 1:1, 2; Luke 1:35; 2 Peter 1:21; John 16:8; John 14:16-18, 26
This is a short summary. The church's full statements are at adventist.org, and our pastors are glad to study any belief with you.