In order to answer your first question, there are three views as to when a marriage begins in God’s eyes. Each view does use Scripture to back it up.

1) Marriage begins with a MARRIAGE CEREMONY. In Genesis 2:21-22 we read, “And the LORD God caused a deep sleep to fall on Adam, and he slept; and He took one of the ribs, and closed up the flesh in its place. Then the rib which the LORD God had taken from man He made a woman. And He BROUGHT HER TO THE MAN.” In most marriage ceremonies today, a father will “give away his daughter” and this is based on “God giving away the bride to Adam.” Another passage of Scripture that supports a formal wedding ceremony is the wedding ceremony in Cana of Galilee which the Lord Jesus and His disciples attended (John 2:1-10).

2) Marriage begins with a LEGAL CONTRACT. We know that in many countries a MARRIAGE LICENSE is required for the man and woman to be “legally married.” The Bible tells us, “Let every soul be subject to the governing authorities. For there is no authority except from God, and the authorities that exist are appointed by God” (Romans 13:1). This surely indicated that God will only sanction a marriage if a legal contract is drawn up and agreed to.

3) Marriage begins when the man and woman CONSUMMATE the marriage by having SEXUAL INTERCOURSE. After God presented the woman to Adam we read, “Therefore a man shall leave his father and mother and be JOINED TO HIS WIFE, and THEY SHALL BECOME ONE FLESH” (Genesis 2:25). In this view the act of sexual intercourse is the “fulfillment of the ONE FLESH principle.” We could say that it is the “final seal of the marriage covenant.”

There may be other views, but these are the three main views that are supported by Scripture. There would surely be some “exceptions to each view,” for 1) when Adam and Eve were married there was “no legal contract”; 2) there are countries that do not require a marriage license; and 3) there are times when a man or woman are unable to engage in sexual intercourse due to physical limitations.

Regarding your second question, there is no scripture that would “limit marriage to believers in the church.” We saw in Genesis 2:25 “Therefore a MAN shall leave his father and mother and be joined to his WIFE.” This surely applies to ALL MEN and ALL WOMEN. In Matthew 19:5 this verse was quoted by Jesus and then He added the words, “So then, they are no longer two but one flesh. Therefore what God has joined together, let no man separate.” There is nothing in these words that would suggest that only believers are married in the eyes of God.

Regarding “divorce laws,” we saw that God meant marriage to be forever, for Jesus said, “What God has joined together, let no man separate.” There is only one EXCEPTION to this which Jesus stated in Matthew 19:8-9, “Moses, because of the hardness of your hearts, permitted you to divorce your wives, but from the beginning it was not so. And I say to you, whoever divorces his wife, EXCEPT FOR SEXUAL IMMORALITY, and marries another, commits adultery; and whoever marries her who is divorced commits adultery.” Men today have passed laws permitting divorce for other reasons (like those in Moses’ day did), but God has not changed His mind. God hates divorce (Malachi 2:16) and He will only permit it in the case of sexual immorality (adultery).  (DO)  (559.5)