By Catholics, specifically. Evangelical Christians don't typically share this belief.
See for example the Catechism of the Catholic Church 1456
> Confession to a priest is an essential part of the sacrament of Penance: "All mortal sins of which penitents after a diligent self-examination are conscious must be recounted by them in confession, even if they are most secret and have been committed against the last two precepts of the Decalogue; for these sins sometimes wound the soul more grievously and are more dangerous than those which are committed openly.
That seems to say mortal sins must be confessed. But I cannot see where it says they can be forgiven.
[1]: https://ucatholic.com/catechism/1864/
[2]: https://www.vatican.va/content/john-paul-ii/en/encyclicals/d... (see 46)
Don’t think that’s true. Murder is considered a grave sin but not one that can’t be forgiven…remember Paul (who wrote several books of the New Testament) was a killer, so was Moses in the Old Testament.
I think the only sin mentioned as unforgivable is “blasphemy against the Holy Spirit” which is very vague.
(Was raised Christian, no longer one).
Edit: Another comment there’s a Catholic-evangelical disagreement on this, so maybe that’s the case.
FWIW I think god is not real, just playing devil's advocate.
That's a reasonable question. Christianity does have an emphasis on forgiveness, which I suspect these prison chaplain programs emphasize, however murder is considered a mortal sin that separates one's soul from god and that cannot regularly be forgiven.
See https://en.wikipedia.org/wiki/Mortal_sin, scroll to 'Actions constituting grave matter' for the list.
My grandmother was devout and spent her last few weeks convinced she was going to hell for something I do not know.