"falling away" in Scripture is use to describe professed believers leaving the faith, leaving Christ, choosing a different faith, a different master.
Iniquity had been working in the followers of Satan since Cain, that was nothing new, it was no mystery.
The mystery of iniquity, is mysteriously perverting truth into a lie, while still maintaining it is truth, it was the falling away from Christ while staying IN Church, creating a different faith and setting up a different Lord and Master.
The path to power was blocked by the rulership of Imperial Rome, but once the Roman Caesar's "made a league" with Christian leaders, then this counterfeit religion started grooming for power and wealth. What started out as a church of God, fell away into a kingdom of Religious intrigue professing Christianity assimilated much pagan thought and customs. Rome's religious professed Christian Pontificus Maximus would assume military might in 538 AD,
Much has already been revealed throughout the medieval times, but the full scope will yet be revealed in the last days. When the true connection between counterfeit Christianity and the dragon (Lucifer) will be revealed.
By reading two prophecies, and putting them together, we shall see the force of this working of Satan, through an agency through which his work has been carried on more successfully and for a longer time than through any other agency in the history of the world.
"How art thou fallen from heaven, O Lucifer, son of the morning! now art thou cut down to the ground, which didst weaken the nations! For thou hast said in thine heart, I will ascend into heaven, I will exalt my throne above the stars of God; I will sit also upon the mount of the congregation, in the sides of the north. I will ascend above the heights of the clouds; I will be like the Most High." Isa. 14:12-14.
This scripture speaks of the fall of Satan and the reason for it; for Lucifer, "the light bearer," by his own course of rebellion became Satan, "the adversary."
"Let no man deceive you by any means; for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition; who opposes and exalts himself above all that is called God, or that is worshiped; so that he as God sits in the temple of God, showing himself that he is God." 2 Thess. 2:3, 4.
The reformers knew this prophecy refers to the papacy, and every one who puts those two scriptures together can see that they represent exactly the same spirit,-I will exalt myself above God.