Dear Hieronymus:

Well, first you need to realize that these people are heretics. It may not be worth dealing with them as their hearts are likely hardened to their heresy.

Keep in mind the admonition of St. Paul:

(Titus 3:10-11)  As for a man who is factious (heretic), after admonishing him once or twice, have nothing more to do with him, knowing that such a person is perverted and sinful; he is self-condemned.  

With that said, here is the Church teaching from the Catechism:

III. ORIGINAL SIN

Freedom put to the test

396 God created man in his image and established him in his friendship. A spiritual creature, man can live this friendship only in free submission to God. The prohibition against eating "of the tree of the knowledge of good and evil" spells this out: "for in the day that you eat of it, you shall die." The "tree of the knowledge of good and evil" symbolically evokes the insurmountable limits that man, being a creature, must freely recognize and respect with trust. Man is dependent on his Creator, and subject to the laws of creation and to the moral norms that govern the use of freedom.

Man's first sin

397 Man, tempted by the devil, let his trust in his Creator die in his heart and, abusing his freedom, disobeyed God's command. This is what man's first sin consisted of. All subsequent sin would be disobedience toward God and lack of trust in his goodness.

398 In that sin man preferred himself to God and by that very act scorned him. He chose himself over and against God, against the requirements of his creaturely status and therefore against his own good. Constituted in a state of holiness, man was destined to be fully "divinized" by God in glory. Seduced by the devil, he wanted to "be like God", but "without God, before God, and not in accordance with God".

399 Scripture portrays the tragic consequences of this first disobedience. Adam and Eve immediately lose the grace of original holiness. They become afraid of the God of whom they have conceived a distorted image - that of a God jealous of his prerogatives.

400 The harmony in which they had found themselves, thanks to original justice, is now destroyed: the control of the soul's spiritual faculties over the body is shattered; the union of man and woman becomes subject to tensions, their relations henceforth marked by lust and domination. Harmony with creation is broken: visible creation has become alien and hostile to man. Because of man, creation is now subject "to its bondage to decay". Finally, the consequence explicitly foretold for this disobedience will come true: man will "return to the ground", for out of it he was taken. Death makes its entrance into human history.

401 After that first sin, the world is virtually inundated by sin There is Cain's murder of his brother Abel and the universal corruption which follows in the wake of sin. Likewise, sin frequently manifests itself in the history of Israel, especially as infidelity to the God of the Covenant and as transgression of the Law of Moses. And even after Christ's atonement, sin raises its head in countless ways among Christians. Scripture and the Church's Tradition continually recall the presence and universality of sin in man's history:

What Revelation makes known to us is confirmed by our own experience. For when man looks into his own heart he finds that he is drawn towards what is wrong and sunk in many evils which cannot come from his good creator. Often refusing to acknowledge God as his source, man has also upset the relationship which should link him to his last end, and at the same time he has broken the right order that should reign within himself as well as between himself and other men and all creatures.
The consequences of Adam's sin for humanity

402 All men are implicated in Adam's sin, as St. Paul affirms: "By one man's disobedience many (that is, all men) were made sinners": "sin came into the world through one man and death through sin, and so death spread to all men because all men sinned." The Apostle contrasts the universality of sin and death with the universality of salvation in Christ. "Then as one man's trespass led to condemnation for all men, so one man's act of righteousness leads to acquittal and life for all men."

403 Following St. Paul, the Church has always taught that the overwhelming misery which oppresses men and their inclination towards evil and death cannot be understood apart from their connection with Adam's sin and the fact that he has transmitted to us a sin with which we are all born afflicted, a sin which is the "death of the soul". Because of this certainty of faith, the Church baptizes for the remission of sins even tiny infants who have not committed personal sin.

404 How did the sin of Adam become the sin of all his descendants? The whole human race is in Adam "as one body of one man". By this "unity of the human race" all men are implicated in Adam's sin, as all are implicated in Christ's justice. Still, the transmission of original sin is a mystery that we cannot fully understand. But we do know by Revelation that Adam had received original holiness and justice not for himself alone, but for all human nature. By yielding to the tempter, Adam and Eve committed a personal sin, but this sin affected the human nature that they would then transmit in a fallen state. It is a sin which will be transmitted by propagation to all mankind, that is, by the transmission of a human nature deprived of original holiness and justice. And that is why original sin is called "sin" only in an analogical sense: it is a sin "contracted" and not "committed" - a state and not an act.

405 Although it is proper to each individual, original sin does not have the character of a personal fault in any of Adam's descendants. It is a deprivation of original holiness and justice, but human nature has not been totally corrupted: it is wounded in the natural powers proper to it, subject to ignorance, suffering and the dominion of death, and inclined to sin - an inclination to evil that is called "concupiscence". Baptism, by imparting the life of Christ's grace, erases original sin and turns a man back towards God, but the consequences for nature, weakened and inclined to evil, persist in man and summon him to spiritual battle.

406 The Church's teaching on the transmission of original sin was articulated more precisely in the fifth century, especially under the impulse of St. Augustine's reflections against Pelagianism, and in the sixteenth century, in opposition to the Protestant Reformation. Pelagius held that man could, by the natural power of free will and without the necessary help of God's grace, lead a morally good life; he thus reduced the influence of Adam's fault to bad example. The first Protestant reformers, on the contrary, taught that original sin has radically perverted man and destroyed his freedom; they identified the sin inherited by each man with the tendency to evil (concupiscentia), which would be insurmountable. The Church pronounced on the meaning of the data of Revelation on original sin especially at the second Council of Orange (529) and at the Council of Trent (1546).

A hard battle. . .

407 The doctrine of original sin, closely connected with that of redemption by Christ, provides lucid discernment of man's situation and activity in the world. By our first parents' sin, the devil has acquired a certain domination over man, even though man remains free. Original sin entails "captivity under the power of him who thenceforth had the power of death, that is, the devil". Ignorance of the fact that man has a wounded nature inclined to evil gives rise to serious errors in the areas of education, politics, social action and morals.

408 The consequences of original sin and of all men's personal sins put the world as a whole in the sinful condition aptly described in St. John's expression, "the sin of the world". This expression can also refer to the negative influence exerted on people by communal situations and social structures that are the fruit of men's sins.

409 This dramatic situation of "the whole world [which] is in the power of the evil one" makes man's life a battle:

The whole of man's history has been the story of dour combat with the powers of evil, stretching, so our Lord tells us, from the very dawn of history until the last day. Finding himself in the midst of the battlefield man has to struggle to do what is right, and it is at great cost to himself, and aided by God's grace, that he succeeds in achieving his own inner integrity.

There are a lot of footnotes in this text, so you will want to look at these paragraphs directly in the Catechism book or online.

Also see the Catholic Encyclopedia.

And, finally, you may want to check out Biblical Evidence for Catholicism. Here is a link to the subject of Original Sin. He has other documents on the subject, so search the site.

You might search Catholic Answers as well.

God Bless,
Bro. Ignatius Mary