b) ONE MAN MARRYING ONLY ONE WOMAN
OLD TESTAMENT: Monogamy is the standard physical relationship that God instituted, when he created Adam and Eve. Monogamy is never explicitly enjoined (commanded), because it is taken for granted, being part of the way God created human beings: "God created man in his own image, in the image of God he created him; male and female he created them." (Genesis 1:27) God did not create them as one male and many females, nor as one female and several males, but as one male and as one female. This is God's creation order. Also in Genesis 2 we read how God made one woman out of one man (from one of his ribs: Genesis 2:21+22). The result is: "Therefore a man shall leave his father and his mother and hold fast to his wife, and they shall become one flesh." (Genesis 2:24) Here there is no talk of a man holding fast to his several wives, but only to his one wife. -- Now the first human was created "in the image of God", i.e. to exemplify deep spiritual realities about God himself. The deepest and most important aspect of being created in the image of God is communication: man and woman can hear and understand what God says to them and they can speak to God, and he understands them. This is what we call prayer. So prayer is rooted in our being created in the image of God. -- The first thing God communicated to Adam and Eve shows us, how they are to reflect God's glory in their physical union. His first communication to them were these commandments: "Be fruitful and multiply and fill the earth …" (Genesis 1:28) Here we have two aspects in which the glory of our triune God is reflected in the human being, which he created in his own image. The first aspect is: a) the physical union of one man and one woman has the power and authority to produce children out of themselves, thus bringing forth new life. This human power is in the image of God the Father, who is the source of all life. Every child born to them is in the image of God the Son, who is the uniquely born Son of the Father. The very notions, which God used in revealing himself as Father and as Son are thus exemplified as an image of himself in the creation order of marriage, in which one man, as father, through union with his wife, procreates a son. -- The second aspect is: b) each man and wife teach their children their language and thus the ability to communicate. They communicate to them the commandments of God: be fruitful and multiply and fill the earth. They have the power and authority not to stay together forever, but to spread out to different parts of the world, ultimately filling the earth. This human power is in the image of God the Spirit, who also goes out into the whole earth, communicating to men the will of God and leading them. -- So with the first two commandments God, who created Adam and Eve, tells them how they should practically exemplify the image of God in their lives. Monogamous marriage thus was created by God to reflect the glory of our triune God himself as Father, Son and Holy Spirit. -- The Old Testament uses the phrase "Adam knew Eve his wife, and she conceived …" (Genesis 4:1) to describe the actual physical relationship between the spouses. Knowing your wife reflects a communion not only on a physical, sexual level, but also on a spiritual, even intellectual level. Here the Bible shows that "becoming one flesh" is much more and deeper than physical union.
NEW TESTAMENT: Monogamy is upheld in the Gospel as rooted in the creation order of God (Matthew 19:3-4). Christ, by calling himself the Son of Man, upheld the creation order of God in Genesis and confirmed it. By proclaiming that he is a descendant of Adam, he said that he was an exemplification of Adam being created in the image of God when he procreated sons, leading to Christ himself, who was born of Mary, a remote descendant of Adam. -- The one-to-one relationship between one husband and one wife is further elevated to becoming a symbol of Christ's relationship to his church: "23 For the husband is the head of the wife even as Christ is the head of the church, his body, and is himself its Savior. … 25 Husbands, love your wives, as Christ loved the church and gave himself up for her. … 31 Therefore a man shall leave his father and mother and hold fast to his wife, and the two shall become one flesh. 32 This mystery is profound, and I am saying that it refers to Christ and the church." (Ephesians 5:23.25.31-32) Here again we have monogamous marriage reflecting deep spiritual realities about God. This time marriage reflects the glory of Christ's relationship to his church: When a man loves his wife, even to the point of sacrificing himself for her, he exemplifies the saving love of Christ, when he died for us to save us from sin, death and the devil. Even at the end of the age this image of God is upheld in the Bible: "Come, I will show you the Bride, the wife of the Lamb." (Revelations 21:9b) -- The result of all this is: In Christianity marriage is a union between one man and one woman; it is instituted and ordained by God as a life long relationship between one man and one woman; and it must be kept pure (Hebrews 13:4).
KORAN: Here monogamy is also allowed. Contrary to the Bible, the Koran does not include an account of the creation of the heavens and the earth in six days, with man being created on the sixth day. Nevertheless the Koran says that humans are creations of Allah. However, the institution and qualification of marriage has been radically changed with respect to the Bible:
The result of all this from Koran is the following: Marriage in Islam is a contract between a Muslim man and one or more women (as long as they are not more than four at the same time); women don’t have to be Muslims but could be Christians, Jews, or Muslims; and the purpose of the contract is to make sexual relationships lawful. (Suras 2:221,235 -- 4:3-4,19-27 -- 5:5)