When did what go wrong?
And what do you mean by "wrong"?
If you mean the so-called "visible church" being infiltrated by the devil it's called the Mystery of Iniquity which began while the apostles were still here:
1 John 4:3 And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world.
BTW, no pope to date has ever denied that Jesus Christ has come in the flesh.
The RCC doctrine of the humanity-deity of Jesus Christ called the Hypostatic Union are shared by Baptist and Catholic theologians and are virtually identical.
Therefore IMO, the pope is not the antichrist (unless he
ex cathedra renounces either the humanity or deity of Jesus Christ destroying the official dogma of the Hypostatic Union).
The RCC will probably be involved as well as the apostate Protestant churches, but a doctrinal overhaul stripping Christ of His glory must first happen.
The Harlot of Babylon represents the ruling entity of the last days, a conglomerate of religion, politics and culture that has been brewing from the beginning.
Revelation 17:18 And the woman which thou sawest is that great city, which reigneth over the kings of the earth.
In Revelation 17:16 the woman is destroyed by the rulling body (ten horns of the beast).
This started early on in the church and Christ gave a series of parables in Matthew 13 which spoke of the "Mysteries" of the kingdom, the infiltration and satanic influence over and in the visible maifestation of the Kingdom of God on earth somtimes called the "visible" church which is presently a "mixed mulititude" of wheat and tares.
It was/is slow, by stealth and deception, so it is impossible to put a date on it as John in his first epistle says it had already begun.
And as certainly as it began it will end:
Matthew 13
36 Then Jesus sent the multitude away, and went into the house: and his disciples came unto him, saying, Declare unto us the parable of the tares of the field.
37 He answered and said unto them, He that soweth the good seed is the Son of man;
38 The field is the world; the good seed are the children of the kingdom; but the tares are the children of the wicked one;
39 The enemy that sowed them is the devil; the harvest is the end of the world; and the reapers are the angels.
40 As therefore the tares are gathered and burned in the fire; so shall it be in the end of this world.
41 The Son of man shall send forth his angels, and they shall gather out of his kingdom all things that offend, and them which do iniquity;
42 And shall cast them into a furnace of fire: there shall be wailing and gnashing of teeth.
43 Then shall the righteous shine forth as the sun in the kingdom of their Father. Who hath ears to hear, let him hear.
HankD