Why would anyone choose to live in the same house or apartment with people or a person in mortal sin, or marry someone in mortal or Original Sin?
Joining one's body and soul in marriage makes a spiritual as well as a physical union. Living in the same house with someone in mortal sin, on purpose choosing to do this, (instead of realizing that people sin and need to go to confession), places oneself in spiritual warfare daily. These thoughts prompted me to realize that most Catholics may not understand the seriousness of spiritual warfare regarding those outside of sanctifying grace.
A parent who is paying for an adult child to be at home can and should insist that while that child is dependent, he or she must abide by religious rules in the house, such as going to confession, Mass, and Holy Communion. One cannot tolerate serious sin daily. When I lived in a lay community, all the people were in sanctifying grace, as these people had to turn away from sin before entering, something which occurred naturally. No one in sin would want to live the rigorous communal life we did.
To pretend there is no a difference in a house where people are practicing Catholics, praying and in sanctifying grace, and a house where this is not the case, simply defies reason. A priest cannot cleanse a house of an infestation as long as the people living there engage in a lifestyle of sin. He can do it once, but the demons will return, worse than before, as the people are living in sin.
Living in mortal sin, obviously, cannot be judged as the same as falling into mortal sin. Living in mortal sin, such as in an irregular marriage, or actively homosexual relationship, or involved in embezzlement or graft or practicing contraception or living in lies and manipulations which are abusive on a regular basis, opens the door for demonic influences beyond one's imagination.
I can walk into a house, as God a long time ago gave me discernment, as He does to all Catholics as part of the gift of knowledge in Confirmation, and tell whether someone is into witchcraft, illicit sex, or whether someone who has lived in a place has been a Mason or whatever. Infestations of demons have become more and more common in houses where mortal sin and even sins which are intrinsic evils have become more common. If an abortion has been done in a house, I can tell. Why would someone want to live in a place full of demons of abortion, murder, illicit sex and so on?
Why would people choose to live with a person or persons who are subjecting one to spiritual oppressions? Why would anyone not choose to live in a house of peace and grace? Why would someone choose companions whom one cannot trust as they have left the road of wisdom and grace?
Many saints write of the necessity for good, holy companions. In Preparation for Death, St. Alphonsus writes in chapters two and four on the evil of bad companions and the blessing of good ones. One must choose close friends who are holy, or at least, in sanctifying grace and working on holiness.
When I was in graduate school at Notre Dame in the early '80s, a friend of mine and I were discussing the really serious problems of one of her roommates who was promiscuous. Finally, as other young women did not want to put up with this, and spoke with her about the line of men she had invited in, the girl moved out. The house became peaceful and a place for study, whereas before there existed chaos, depression, and anxiety. The poor young woman had been encouraged to have sex with men by her own parents as a teen, and could not break the habit. As a non-practicing Catholic, she was not turning to grace for the help needed to break away from sin. She would not listen to those of us who tried to help her out of her desperate lifestyle. But, one cannot allow such lifestyles to take over a Catholic environment of prayer and peace, if someone is not willing to change. The other women had determined the household rules and she would not abide by these.
I myself asked the sister in charge of the women's dorms and townhouses to live with nuns while I was a grad student at ND, as I got tired of fornication and the resulting spiritual warfare. Thankfully, my request was granted.
There are, of course, other serious sins, such as maliciousness, which can infect a house. A woman I know had to divorce her husband many, many years ago as he insisted on dubious and criminal business deals. He was soaked in avarice, and left the Church. He refused to repent and the chaos in the house affected the children. She was fortunate to be able to move away with the children safely. However, the children were affected and several have fallen away. Such are the sins on the consciences of parents.
I think of St. Padre Pio refusing to hear the confession of one woman who walked into a room where there were many people waiting to go to him for the sacrament. As the woman, dressed very well, rather middle class, came in, the saint rushed in and said, and I paraphrase, "I shall not hear your confession until you really repent. Your son is in hell because of you."
As a parent, I remember these words. I do not want to be held accountable for any reason for the sins of my son. But, many parents will be held accountable, for laxity, or obvious sin, or sins against the children, such as incest which can lead a person to be an abuser themselves, (although, no one is doomed to sin, of course), and so on.
Living in sin creates a place for demons to reside, and demons like to attach themselves to places, as well as to people.
To pretend mortal sin does not open one up to more evil is to ignore Church teaching and common sense.
Perhaps a review of mortal sin is needed here. From the CCC:
1855 Mortal sin destroys charity in the heart of man by a grave violation of God's law; it turns man away from God, who is his ultimate end and his beatitude, by preferring an inferior good to him.
1858 Grave matter is specified by the Ten Commandments, corresponding to the answer of Jesus to the rich young man: "Do not kill, Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honor your father and your mother."132 The gravity of sins is more or less great: murder is graver than theft. One must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger.
1859 Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God's law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart133 do not diminish, but rather increase, the voluntary character of a sin.
1860 Unintentional ignorance can diminish or even remove the imputability of a grave offense. But no one is deemed to be ignorant of the principles of the moral law, which are written in the conscience of every man. The promptings of feelings and passions can also diminish the voluntary and free character of the offense, as can external pressures or pathological disorders. Sin committed through malice, by deliberate choice of evil, is the gravest.
1861 Mortal sin is a radical possibility of human freedom, as is love itself. It results in the loss of charity and the privation of sanctifying grace, that is, of the state of grace. If it is not redeemed by repentance and God's forgiveness, it causes exclusion from Christ's kingdom and the eternal death of hell, for our freedom has the power to make choices for ever, with no turning back. However, although we can judge that an act is in itself a grave offense, we must entrust judgment of persons to the justice and mercy of God.
One of the common statements on this blog is that there is not middle ground in the spiritual life-one is either for God or against Him. One's house either becomes a sanctuary or a war-zone of spiritual warfare.
These situations I am describing are not the same as a man or woman converting or reverting after a marriage outside the Church which must be regularized. If it can be, then the Catholic has a commitment to the spouse to bring that person into grace.
If the spouse refuses to regularize the marriage, one must make a choice with guidance from a holy priest. The Catholic spouse in a regular, legal and sacramental marriage can lead the other one to God, of course, The main reason for marriage is to lead each other to heaven.
I know many spouses who have converted their husbands or wives, praise God. But, I know many more where the unbelieving spouse has not converted. It will be through the merits and love of the practicing Catholic if that person is saved. One can turn to St. Paul for understanding on this point.