I agree that apostasy doesn’t happen overnite. But the theory of the great apostasy has to happen at a particular point in time when every person here in this world didn’t believe in Christ and HIS teachings. Could you pin point that date? I don’t think so because it never happened. JS needed this theory so he could be considered the restorer of the Church of Christ because if there was no Total apostasy there would be nothing to restore.
When it comes to your reference to Pope Gregory the Great I think you need to do a little bit more research. Pope Gregory the Great reigned as pope during the 6th century. He died 604 AD. The first crusade was on 1069 under the reign of Pope Urban II and this was just a response to the need of the Church in the East when the Byzantines were being sacked by the turks. How then can Pope Gregory invent the crusades.
Gregory the great never popularized the worship of statues or relics because up to this point the Catholic church hasn’t taught that. The worship of statues is a total misconception of the Prostestant church. Although we kneel in front of statues, it is suppose to just aid us in our prayers. To remind us of the lives of these holy men and women who have gone before us so that we can imitate them.
The doctrine of Purgatory is not something new. The Maccabeans believed in this although the word purgatory is not explicit in the Bible(2 Maccabees). Don’t try to find 2 Maccabees in your Bible 'coz you’ll never find it. But just in case you want more proof look at 1 Cor 3:14-15. Here is a better description of purgatory quoted from
newadvent.com
Purgatory (Lat., “purgare”, to make clean, to purify) in accordance with Catholic teaching is a place or condition of temporal punishment for those who, departing this life in God’s grace, are, not entirely free from venial faults, or have not fully paid the satisfaction due to their transgressions. The faith of the Church concerning purgatory is clearly expressed in the Decree of Union drawn up by the Council of Florence (Mansi, t. XXXI, col. 1031), and in the decree of the Council of Trent which (Sess. XXV) defined: “Whereas the Catholic Church, instructed by the Holy Ghost, has from the Sacred Scriptures and the ancient tradition of the Fathers taught in Councils and very recently in this Ecumenical synod (Sess. VI, cap. XXX; Sess. XXII cap.ii, iii) that there is a purgatory, and that the souls therein are helped by the suffrages of the faithful, but principally by the acceptable Sacrifice of the Altar; the Holy Synod enjoins on the Bishops that they diligently endeavor to have the sound doctrine of the Fathers in Councils regarding purgatory everywhere taught and preached, held and believed by the faithful” (Denzinger, “Enchiridon”, 983). Further than this the definitions of the Church do not go, but the tradition of the Fathers and the Schoolmen must be consulted to explain the teachings of the councils, and to make clear the belief and the practices of the faithful.
I hope this helps you get a better understanding of Catholic doctrine.