Genesis tells of man and woman made in God’s image, the blessing on them, and the words that a man shall cleave to his wife and they shall be two in one flesh. Christ returns to these words in the Gospel as the original plan. It bears on the teaching that marriage is instituted by God, the union of two in one flesh.
And God created man to his own image: to the image of God he created him: male and female he created them. And God blessed them, saying: Increase and multiply, and fill the earth, and subdue it, and rule over the fishes of the sea, and the fowls of the air, and all living creatures that move upon the earth.
And the Lord God said: It is not good for man to be alone: let us make him a help like unto himself.
And Adam said: This now is bone of my bones, and flesh of my flesh; she shall be called woman, because she was taken out of man. Wherefore a man shall leave father and mother, and shall cleave to his wife: and they shall be two in one flesh.
