I've seen some of them - including some pastors - use the text in 2 Corinthians 6 about being yoked together with unbelievers to mean that adventists shouldn't marry non-advenists. But a Christian of another denomination is also a believer so this doesn't make sense to me. Believer in the Word means someone who has put their faith in Jesus Christ, not a specific denomination or set of smaller beliefs - it's about the one big belief of the deity of Jesus and the salvation he offers. Why do some adventists think that marrying a non-believer is a sin and something God can't approve of? There weren't even any adventists in Jesus' day - they didn't exist until 1880 or something, yet God approved of these people who came before and of theur unions in marriage. Jesus turned water into wine for a non-adventist wedding.