Dear Father:

Thank you for this question. Many people, including priests, do not understand what is meant by "ancestral demonization."

Baptism, of course, does remove Original Sin, and any actual sin, and does contain a simple exorcism. But, this does not guarantee that the person will be free from demonization any more than does the Sacraments of the Confirmation, Eucharist, and Confession guarantee that.  Indeed Confession and the Eucharist are two of the most powerful weapons against the devil, but it does not guarantee deliverance. Even solemn exorcisms do not guarantee deliverance. There are cases in the Church in which solemn exorcism were not successful.

And, of course, the forgiveness of sin does not necessarily remove the "consequences" of sin. That is why we have a prayer in our catalog that I recommend to everyone to "take back the ground" Satan has stolen because of the sin they committed. The sin is forever removed and the soul cleansed of the sin when Confessed (i.e. 1 John 1:9), but the consequence of the sin, either spiritual or material consequences may remain.

For example, if I drive drunk and run into someone causing them to be paralyzed, I can be forgiven by God and by the victim, by the consequence of the poor man made crippled remains. In the same way, after one's sin has been forgiven, the consequences of demonic harassment that caused by the sin may still remain and must be dealt with. Thus, after Confession the person needs to kick the devil off their property, as-it-were. This sort of language about "taking back property" is military language. St. Paul refers to the attacks of the devil in warfare terms. This is, after all, this is a real warfare, just as real as that in Afghanistan, only in the spiritual realm. 

The "sins of the father" passed on from generation to generation is explained in three possible ways:

1) First possibility in in genetic traits, of course. This is beyond anyone's control. But, while a person may have a genetic propensity, it is usually environment that nurtures that propensity. For example, a person may inherit the genes that are responsible for aggression. It is environment that will mold that genetic propensity into aggressive behavior, or to a more positive assertive behavior.

2) The second cause is what I call environmental heredity. This refers to the bad (or good) behaviors children learn from watching their parents, and the psychological effects of how the children are treated and what they experience in life. These factors can have long-ranging effects that can last a lifetime.

Neither of these two factors require any demonic involvement at all. But, demons can exploit environmental issues to help make things worse. Demons are opportunists. If they see a problem that already exists, they will happy to exploit that vulnerability to make the problem worse than it would have been otherwise. 

3) The third possibility is what is called "ancestral demonization". This phenomena suggests that some family member, a parent, grandparent, great-grandparent, etc., has committed some sin, or lived some sinful life, that has attracted the devil to them and to the family line. The family gets placed on the devil's radar, as-it-were.

Demons are not "inherited" like the genetic traits in our genes, but demons, once they have the family on their "radar", may harass the family through the generations. This does not guarantee that each and every generation will have demonic problems, nor that each and every person from from the generations will be harassed by demons.

Ancestral Demonization(or a "family curse") simply means that, for whatever reason, the devil has been attracted to the family and thus he will look for opportunities to intrude upon that family in some way. He may or may not succeed.  Our Prayer Of Renunciation Of Ancestral Sins is designed to break this cycle, or close that door, to remove the family member from that radar. The worse cases are when parents or grandparents actually call down evil upon their prodigy. In those cases that do not require an exorcism, not only is the above prayer is useful, but also Breaking Personal Curses and Spells. One of the worse cases that lead to possession and a formal exorcism of an innocent girl is most famous documented case in Earling, Iowa in 1928. This case is recounted in the book, Begone Satan. 

This analysis I have given here has been gained by the experiences of exorcists and deliverance counselors.  Other than the paragraphs in the Catechism (nos. 517, 550, 1237, 1673), and the Rite of Exorcism itself, there is no actual Church teaching that specifically talks about this particular question, to my knowledge. Rather, this information is derived from many sources, including the writings of the Saints on matters pertaining to demonology, other writers on the theology of angels and demons, and the clinical experiences of exorcists (such as Fathers Amorth, Fortea, and others) and deliverance counselors.

In his book, An Exorcist Tells His Story, Father Amorth writes about curses. He said:

Curses invoke evil, and the origin of all evil is demonic. When curses are spoken with true perfidy, especially if there is a blood relationship between the one who casts them and the accursed, the outcome can be terrible. The most common instances that I have encountered involved parents or grandparents who called down evil upon children and grandchildren. 

The fact that family curses exist from generation to generation is clinically well known by those of us with experience in this field and is perhaps the most common source of the demonization of an innocent person.

God Bless,
Bro. Ignatius Mary