It started after The King departed.
The first was the return to the heretical order of priesthood that was so entrenched in the temples.
It snowballed from there.
I believe Paul shows us the point of the "falling away" it is clearly the establishment of organized religion based not on the Holy Spirit, but on the carnal religious system.
Ac 20:21 Testifying both to the Jews, and also to the Greeks, repentance toward God, and faith toward our Lord Jesus Christ.
22 And now, behold, I go bound in the spirit unto Jerusalem, not knowing the things that shall befall me there:
23 Save that
the Holy Ghost witnesseth in every city, saying that bonds and afflictions abide me.
24 But none of these things move me, neither count I my life dear unto myself, so that I might finish my course with joy, and the ministry, which I have received of the Lord Jesus,
to testify the gospel of the grace of God.
25 And now, behold, I know that ye all, among whom I have gone preaching the kingdom of God, shall see my face no more.
26 Wherefore I take you to record this day, that I am pure from the blood of all men.
27 For I have not shunned to declare unto you
all the counsel of God.
28 Take heed therefore unto yourselves, and to all the flock,
over the which the Holy Ghost hath made you overseers, to feed the church of God, which he hath purchased with his own blood.
29
For I know this, that after my departing shall grievous wolves enter in among you, not sparing the flock.
30
Also of your own selves shall men arise, speaking perverse things, to draw away disciples after them.
31 Therefore watch, and remember, that by the space of three years I ceased not to warn every one night and day with tears.
32 And now, brethren, I commend you to God, and to the word of his grace, which is able to build you up, and to give you an inheritance among all them which are sanctified.
33 I have coveted no man's silver, or gold, or apparel.