In short
God the Son
God the eternal Son became human in Jesus Christ. Through Him all things were created, God's character was revealed, salvation was won for us and the world will be judged. He was truly God and truly man, lived without sin and will return.
Read it in the Bible
John 1:1-3, 14; Colossians 1:15-19; Hebrews 2:9-18; Philippians 2:5-11
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.