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You’re missing the point.  There is no unanimous opinion on the EXACT specifics of the falling away.  But there is agreement, generally. 

1.  St Paul says that the Antichrist will not come before the great falling away. 

2.  It is unanimous (or near unanimous) that the Church Fathers said the 3rd and Final Holy Roman Empire will exist before Antichrist.  This is a direct implication that the whole world will be catholic.  (Which it’s not today and NEVER has been).

3.  This 3rd catholic empire is spoken of countless times in prophecies, consistently given to us in almost every century, going back to the 400s.

4.  The Apocalypse speaks of 10 kings and 10 kingdoms, which lines up with the return of the Empire.  It describes these 10 kings being coerced and conquered by Antichrist. 

Add all this up, and the great “falling away” is the fall of the 3rd Holy Roman Empire and the fall of Catholicism (generally speaking) back into paganism and worship of Antichrist. 

What we see now is the “falling away” of Western Christianity to atheism.  What will happen in the future is the “falling away” of the entire Christian world to Antichrist. 

Our times are a prefigurement of what is to come.  Our Lady will triumph over Fɾҽҽmαsσɳɾყ now.  In the future, Our Lord will have the victory over Antichrist. 

That’s why Our Lady said that after Russia is consecrated, “a certain period” of Peace will be granted to the world.  Because Her victory is not over the actual Antichrist, but only over his agents who prefigure his coming. 

Christ will have the final victory over Antichrist and He will be the one to convert the Jєωs before the end of time.  This will be His reign and the Social Kingship, when the Jєωs convert and Scripture is fulfilled. 

All this lines up with the Apocalypse and the many, many prophecies. 

St Jerome (4th cent)...in his commentary on the Book of Daniel:

"Therefore, let us state what all the Ecclesiastical writers [omnes scriptores ecclesiastici] have passed down [tradiderunt]: At the consummation of the world, when the Kingdom of the Romans has been destroyed, when ten kings shall have divided the territory of the Romans between themselves, an eleventh shall rise to a small kingdom, who when he shall have overcome three of the ten kings, i.e. the kingdom of the Egyptians, of the Africans and of the Ethiopians and consequently as we learn more manifestly - whom he shall have killed, the other seven kings shall submit their necks to the victor [the eleventh king].

St Jerome says that (ALL Ecclesiastical writers agree...i.e. unanimous) - the "Kingdom of the Romans" will be destroyed before antichrist arises to power.  There is currently not a Roman Empire.  Thus, we can safely infer that there will be a 3rd and final Roman Empire, as many, many prophecies, Saints and Church Fathers teach.

Since this Roman Empire will be catholic, then it will be a Holy Roman Empire.  Since this Empire will be global, then we can infer that the whole world will be catholic (legally and politically).

Also, let's remember that St Jerome died in the early 400s.  He composed the Bible and was considered the most learned of all Latin Fathers.  He would've been able to read all the opinions of the Church Fathers and know what was the consensus and what wasn't.  Those who lived prior to the peace of Constantine in 325 did not have the luxury to "compare notes" as the Church was fractured and under various persecutions.  St Jerome was able to view all writings of the Church Fathers together, at once.  Those who lived previously did not have this ability.

the Faith would be heard throughout the world, not that "the whole world will be catholic [sic]."

It's a little bit of potato, potatoe.  One could debate on what does the being "heard" mean?  If we go by history, then as Christ's time was a peaceful one for the world, then "hearing the Faith" means one had the opportunity, in a foreign land, to peacefully accept the Faith and not be persecuted.  In our present day, many countries are hostile towards Christianity so while many can "hear" the Truth, they don't have the opportunity to peacefully accept it, nor peacefully practice it.  (Many muslims/chinese/africans are under threat of death if they accept the Faith.)  My opinion is that there will be a time (i.e. the 3rd Holy Roman Empire) when there will be peace and thus the "hearing" of the Faith will be easier and genuine.