In a secret way, leaven was introduced into that which was intended for YAHWEH. False teachers brought corrupt and unhealthy elements into the teaching that had been handed down by the apostles. This was already the case in Paul’s day and we find it again in the letters.
Christianity
Ecclesias that came into being organically as the body of Christ and came together around the Word of the Head, Christ Jesus, were organized. They became institutions and churches, embedded in a system and the churchgoers were subjected to dogmas and church doctrine.
invalid
The Word of God became invalid and invalidated (Matt.15:6), because authority was attributed to forms and other human writings and thereby placed them above the Bible. Church doctrine became leading.
another gospel
‘Grace alone’ became: also wanting to be justified by the law (Gal.5:4) and one was transferred to another gospel (Gal.1:6).
The testimony of Scripture that there is one God (Deut.6:4; 1 Cor.8:6) became disguised polytheism and one introduced the doctrine of the Trinity.
Destructive groups would be ‘smuggled in’ (2 Pet. 2:1) and at first some would turn away from the faith and turn to teachings of demons (1 Tim. 4:10), but soon they would no longer tolerate sound doctrine at all (2 Tim. 4:3).
everything is from Him
We could give countless more examples of how Christianity has been leavened with false teaching. After all, the dough is completely leavened.
The most fatal deviation is that people do not acknowledge God as God and do not glorify and thank Him as God (Rom.1:21). God is the Creator and Placer of all things (Prov.16:4; Isa.45:7; Rom.11:36), but that He is also the Creator of evil and of Satan, is reasoned away.
Scripture teaches that everything is from Him, through Him, and to Him (Rom. 11:36). God is the Maker and Creator of everything. And He brings everything where He wants it: (back) to Him. But people do not believe that He is the Savior of all people (1 Tim.4:10) and reconciles all creation to Himself (Col.1:20).
from outside and from inside
The parable of the mustard seed and the parable of the leaven have a similar meaning. In the parable of the mustard seed, it speaks mainly of the outside. The mustard seed would grow into something that may look impressive on the outside, but which is degenerate and unnatural.
The parable of the leaven speaks of how that which is intended for YAHWEH would be leavened, from within. Starting with a little bit of false teaching, but it would penetrate through the entire dough. Leaven makes the flour rise and puffs it up, without increasing its weight. Christianity has become all leavened and puffed up, so that it seems to be something, but it has no substance and is corrupt.