Upvote:3
Fear (unlike compulsion/violence) doesn't take away involuntariness. The person in your scenario chooses to deny Christ, not in itself but only on account of avoiding what he fears.
St. Thomas Aquinas, Summa Theologica I-II q. 6 a. 6 ("Whether fear causes involuntariness simply?") ad 1:
the will does not consent, but is moved entirely counter to that which is done through compulsion: whereas what is done through fear, becomes voluntary, because the will is moved towards it, albeit not for its own sake, but on account of something else, that is, in order to avoid an evil which is feared. [β¦] what is done from compulsion, the will does nothing inwardly; whereas in what is done through fear, the will does something.
You are correct that sin must be voluntary. One cannot be forced/compelled against his will to sin.
But does the persion in your scenario commit a venial or a mortal sin? Discussing "Whether fear is a mortal sin?" (Summa Theologica II-II q. 125 a. 3 co.), St. Thomas Aquinas writes:
fear is a sin through being inordinate, that is to say, through shunning what ought not to be shunned according to reason. Now sometimes this inordinateness of fear is confined to the sensitive appetites, without the accession of the rational appetite's consent: and then it cannot be a mortal, but only a venial sin. But sometimes this inordinateness of fear reaches to the rational appetite which is called the will, which deliberately shuns something against the dictate of reason: and this inordinateness of fear is sometimes a mortal, sometimes a venial sin. For if a man through fear of the danger of death or of any other temporal evil is so disposed as to do what is forbidden, or to omit what is commanded by the Divine law, such fear is a mortal sin: otherwise it is a venial sin.
See the related question: "Can 'feelings and passions' make an act involuntary?"
Pope St. Marcellinus was deposed for having offered incense to idols out of fear but later recanted and was re-elected pope.
Upvote:6
Peter denied Jesus three times within a gap of an hour (Lk. 22 : 56-60) . But he repented immediately ( Lk 22: 62) and was later chosen by Jesus as the leader of His Church. See that Jesus gets it confirmed from Peter not once, but three times as if a taunting reminder of the latter's `ditching' of his Master (Jn 21: 15-17) that he really loved Jesus . God who can see the inner thoughts of His children, is the only one who can judge the situation on merits and decide whether the one who is being persecuted commits a venial sin or mortal sin by denying his faith in the Lord.