Dear Christian:

St. Jerome's letter, The Perpetual Virginity of Blessed Mary, is a letter to refute the heresy of Helvidius, who claimed the Blessed Mother was not perpetually virgin and bore other children.

The impiety of Helvidius refers to his lack of respect toward the Scriptures, God, and to our Blessed Mother for proposing an idea contrary to Scripture and to the Church teaching.

Impiety means, "lack of reverence or proper respect toward God, the Saints, or Virtue." The word can also mean a lack of "dutifulness." Helvidius was not dutiful to a proper understanding of the Greek when he interprets Scripture to allegedly prove his point.

St. Jerome says it himself of Helvidius in paragraph number 1:

I was requested by certain of the brethren not long ago to reply to a pamphlet written by one Helvidius. I have deferred doing so, not because it is a difficult matter to maintain the truth and refute an ignorant boor who has scarce known the first glimmer of learning...

Impiety and heresy are not the same thing. Heresy is the worse sin. Heresy is such a major crime against the Faith that it incurs an automatic excommunication. Impiety, in-an-of-itself, does not incur such a penalty.

One can be impious and not a heretic. In this particular case the impiety of Helvidius happened to deal with the heretical notion he held. Each time a person says the name of God in vain, for example, he is impious, but not a heretic.

Heresy is objectively a grave sin. Impiety may or may not be grave depending on the nature of the disrespect and irreverence.

Any grave sin, unconfessed, makes a person's soul at risk.

As to your comment, "one that denies an essential article of truth would see no need to repent", such a person may not see a need to repent if his heart is so hardened against the faith that he refuses. Even so, such a person is in danger of losing his soul in hell. There is no excuse for any Catholic to not see the need to repent. The definition of heresy is found in Canon Law 751:

Heresy is the obstinate denial or obstinate doubt after the reception of baptism of some truth which is to be believed by divine and Catholic faith...

In order for a someone to be a heretic he must be a Catholic and then obstinately deny or doubt some point of the faith required for belief. In most cases, such people know the faith, they are not ignorant of it, but they choose to deny it or to obstinately doubt it.

Protestants, on the other hand, are in what is called, "material heresy" because they deny certain aspects of divine revelation, but do so in ignorance. Material heresy means that some of their views are technically heretical, but they bear no personal responsibility for those views because they were never taught the correct teaching in the first place. Such persons, according to Church teaching, are not risk for damnation because they lack belief on those points "through no fault of their own."

God Bless,
Bro. Ignatius Mary