Obviously they were guilty of mortal sin.
It was a formal act of apostasy which they were obliged to resist even unto martyrdom.
I get the distinct impression some here believe their religion ought not inconvenience them, much less ever require martyrdom.
I was researching to refute 2Vermont, and stumbled across this from St. Thomas Aquinas (Q. 125, Art. 4,
Secunda Secundae):
"Now the
evils of the
soul are more to be feared than the
evils of the body. and
evils of the body more than
evils of external things. Wherefore if one were to incur
evils of the
soul, namely
sins, in order to avoid
evils of the body, such as blows or death, or
evils of external things, such as loss of money; or if one were to endure
evils of the body in order to avoid loss of money, one would not be wholly excused from
sin. Yet one's
sin would be extenuated somewhat, for what is done through fear is less
voluntary, because when fear lays hold of a man he is under a certain
necessity of doing a certain thing. Hence the
Philosopher (Ethic. iii, 1) says that these things that are done through fear are not simply
voluntary, but a mixture of
voluntary and involuntary."
https://www.newadvent.org/summa/3125.htm Consequently, I modify my own opinion quoted above:
It would certainly be grave matter, but may not be mortal sin.
Concedo.