whywas a mortal sin punishable by Hell prior to 1965 and now it is no sin at all? How can a mortal sin be dismissed or changed into a non-sin?
You seem to be confused about what exactly the sin was. The sin was disobedience to the law of the Church, which is still a sin. The law may change, but we are still bound to follow the law, whatever it is.
Eating meat was never the sin.
The eating of meat was never a mortal sin. (I mean, think about it. How could eating meat only be sinful on Friday, but OK other days?)
The DISOBEDIENCE in eating the meat on a day one was asked not to do so was the sin, and even then, the conditions for mortal sin had to be met: Full knowledge, grave matter, and full consent.
Perhaps you are not aware that to this day, the NORM of the Church is to abstain from meat on all Fridays, as a penance to show our sorrow for Christ’s suffering. Places such as the US have received an indult to limit abstinence only to Lenten Fridays. An indult means that for pastoral reasons (the US bishops in this case), the NORM is “ABROGATED”.
So to this day, the mortal sin would be that a Catholic, knowing that the Church requested them, through the Holy Spirit, to make a little sacrifice for God on certain days, knowing that this was not something ‘light’, would freely and fully say, “I WILL eat the meat, knowing it is wrong to do so, because I only care about myself. God doesn’t mean enough to me to deprive myself for Him”. . .
A Catholic who accidentally ate meat is not committing a mortal sin. A Catholic who truly does not know why this request from the Church is truly something God Himself gave us through the Holy Spirit, and should not be ignored, is not committing a mortal sin. One MUST have the knowledge, one must accept that this is truly something gravely wrong, and one must have full informed consent.
Because the whole point is OBEDIENCE TO GOD. DISOBEDIENCE (fully known, grave in matter, and freely chosen) is the mortal sin, not ‘eating meat on Friday’.
I hope that helps you understand.
Because the Church regulation changed.
As an analogy:
If the posted speed limit on Hwy 102 last year was 55 mph and you were caught going 70 mph, you would be guilty of breaking the speed law and have to pay the penalty.
If this year the speed limit on Hwy 102 was changed to 70 mph, you could travel at 70 mph and not be guilty of breaking any law and subject to penalty.
The Church commandment calling for abstinence from meat on Fridays was not enacted because there is something intrinsically immoral about eating meat.