CCC 1864 is also standard teaching throughout all Christendom.
May I add my own peculiar observation though.
A mortal sin is forgivable if properly confessed, sincerely regretted and fellowship with God is sought by the offender.
Also a mortal sin does not have to specifically be an offense against the Holy Ghost.
Blasphemy against the Holy Ghost is one particular mortal sin (there are many others), it is the only one that our Lord declares irreversible as He stated in:
Matthew 12:31-32 Wherefore I say unto you, All manner of sin and blasphemy shall be forgiven unto men: but the blasphemy [against] the [Holy] Ghost shall not be forgiven unto men. And whosoever speaketh a word against the Son of man, it shall be forgiven him: but whosoever speaketh against the Holy Ghost, it shall not be forgiven him, neither in this world, neither in the [world] to come. (please read the paragraph preceding v31 for context so that you may relate it to CCC 1864.
Many Christians read these particular verses and become afraid of having committed an unforgivable sin.
The sincere reading of scripture and the fear of offending God are good evidence that this sin has not transpired.
This is just my opinion but I believe it will test well; I don’t believe it is possible for a confirmed believer in Jesus Christ to commit this unforgivable sin, the Holy Ghost Himself will protect them. Our Lord did not give sufficient details to make it possible for anyone to write a recipe for committing this sin, He loves us too much to allow such a temptation.