"WHY is a calendar such a big deal you bigoted fanatics? Have you no scope for economia? Are you greater and more strict than our fathers?"
This is a common objection usually raised by New Calendarists who haven't much researched what anathema IS...
Here is why, in simple language, it is such a big deal and in fact a betrayal of the faith:
The Church has anathematized the Papal Gregorian calendar. Both the Paschalion and the Menologion.
Now, that which is anathematized is cursed. That means to place ourselves willfully under anathema is to invite communion with Satan, rejection of Christ, and to fall outside the Church.
Now, given how serious the anathema is, there is absolutely zero legitimate use for "economia" in regard to that which is anathematized. That which is anathematized is under the dominion of Satan, and there can be no fellowship or communion between Christ and Satan.
But the Calendar reform was initiated precisely for this reason: as a way to prepare us for being ready and able and willing to "tolerate" the setting aside of the anathema. It was instigated precisely to foster the notion that the Orthodox and heterodox together constitute one Church of Christ. In other words, if the Church was a fortified city, and the liturgical calendar was the outer wall, the barrier that was the first line of defense, that which clearly, and in the minds of the faithful, distinguished them from the world and from the heretics in it, then it's overthrow must be considered a betrayal of the faith.
We would say, "But wait, it was only a calendar! Nobody denied the dogmas of the faith!" This is missing the point.
If a person sneaks out at night to the outer wall of the city and gently unlocks the gate, is this such a terrible thing? I mean, it's such a small lock, the key is so tiny, it's such a simple action: just lift the latch and slide it over and sneak back to bed. It's not so significant is it?
But in the morning, what do we awake to? The enemy of our souls is crossing the square and attempting to march on the outposts that guard the citadel of the King!
So, no, it is not a small thing. The man who sneaks out at night and lifts the latch to the city gates is no less guilty of treason than the one who steals the guards sword and slaughtered the princes of the city.
Both have placed themselves under anathema, both have fallen into heresy, and both have ceased to be members of Christ and his Church.
And look at what we see: the New Calendarists are all at the forefront of ecumenism, they all want to set aside the anathemas of Chalcedon for unity with the Monophysites. The Old Calendar ecumenists of Russia and Jerusalem lag slightly behind, but are nevertheless perishing because of their communion with heresy. Now the Patriarch of Moscow signs joint declarations with the Pope.
And you are not helping, Conservative New Calendarists! Foolishly you hope to battle in the square but insist that the outer gate be left open! You insist on retaining the anathema of the New Calendar for yourself and yet claim to fight ecumenism! No, you are, perhaps unwittingly, serving its agenda and will never see victory! You have not yet in nearly 100 years WON a single battle!
There is only one solution: Denounce the anathematized, confess the faith, and commune solely with those who refuse any communion with innovation or ecumenism.
Shut the gates, heal the sick, bury the dead, regroup, and pray.