The essential properties of marriage are unity and indissolubility.
Its unity consists in the fact that the husband cannot have another wife so long as his wife is alive, and the wife cannot have another husband so long as her husband is alive. These passages from the Scriptures confirm this understanding:
“And there came to Him the Pharisees tempting Him, and saying, ‘Is it lawful for a man to put away his wife for every cause?’ Who answering, said to them, ‘Have ye not read, that He who made man from the beginning, made them male and female? And He said: for this cause shall a man leave father and mother, and shall cleave to his wife, and they two shall be in one flesh’. Therefore, now they are not two, but one flesh. What therefore God hath joined together, let no man put asunder.’ They said to Him, ‘Why then did Moses command to give a bill of divorce, and to put away?’ He saith to them, ‘Because Moses by reason of the hardness of your heart permitted you to put away your wives, but from the beginning it was not so. And I say to you, that whosoever shall put away his wife, except it be for fornication, and shall marry another, committeth adultery; and he that shall marry her that is put away, committeth adultery’” (Mt. 19:3-9).
“For the woman that hath a husband, whilst her husband liveth is bound to the law. But if her husband be dead, she is loosed from the law of her husband. Therefore, whilst her husband liveth, she shall be called an adulteress, if she be with another man: but if her husband be dead, she is delivered from the law of her husband; so that she is not an adulteress, if she be with another man” (Rom. 7:2-3).
“But to them that are married, not I but the Lord commandeth, that the wife depart not from her husband. And if she depart, that she remain unmarried, or be reconciled to her husband. And let not the husband put away his wife” (I Cor. 7:10-11).