Believed in what? That Jesus said He was the Son of God and shown in Paul that He Was/is God, yet the God of Abraham, Isaac and Jacob. The OT God was/is ONE, hence Jesus was Him, yet we see at the Baptism three "aspects" of this ONE God, hence the ONE God is in three -- Father, Jesus and the Holy Spirit
Hence a Christian is a disciple of Christ, believing that Christ is God, part of the ONE God, 100% God and 100% man
The authority is Jesus Christ Himself, the Word of God who says that
That is just one person’s interpretation of scripture, that is no diferent than a personal opinion. That is not an authority.
Hint: To a Roman Catholic, an authority would be the Roman Catholic Church.