the joining of a man and woman as one flesh is considered natural law so I do not believe that you must be Christian to be joined. Once you decide to be joined with a person then, according to the text, you are by natural law joined as one flesh. According to Jesus' statement all first marriages (100%) were joined by God.
Kind of a strange footnote on this, I am in my second marriage. I was a non-believer throughout my life and my first husband and I divorced after a rocky 7 year marriage. My current husband is Christian and through him I have come to know Christ and accept Christ as my Lord and Savior. With that being said, his divorce was because of adultery (ex-wife cheated) so according to the Bible his divorce was considered "ok" but what about me? If I was not a believer before am I still wrong in the eyes of God and have I caused my current husband to sin because he married me? This has bothered me (not enough to leave my faith or my husband but hard just the same) and I get several different interpretations of what the bible says about this.
I would love to hear the thoughts of the group. If I accepted Christ as my Lord and Savior AFTER my divorce, am I cleansed of that sin?
Sorry for the rant but thanks for reading!