Does baptism cleanse a person from an unbiblical marriage?
No. Baptism does not change the fact that we are in sin if we continue to participate in that sin. A sin is a sin, regardless of our relationship with Christ, and baptism does not mysteriously make an adulterous relationship a healthy and acceptable one. Even non-Christians are responsible to Christ for their sins, for we are told in Acts 2:38 that repentance is a necessary prerequisite to salvation. We must change the sinful conditions in our lives upon conversion to Christ. After all, Paul writes, Let him who steals, steal no longer in Ephesians 4:28. Can we not make this lesson applicable to all sins? Let him who commits adultery, commit adultery no longer.