The largest of mustard plants, even under ideal conditions, can grow only to a height of about 15 feet. Luke 13:19 describes it as "a large tree," yet the natural mustard plant is not "a large tree" by any stretch of the imagination. All varieties of the mustard family, which are herbs, have thin, pulpy—not woody—stems and branches. It is nothing like a tree.
This mustard plant did something abnormal by growing beyond its design parameters; it became larger than what God designed as normal. What is this large mustard tree in which, apparently, demons are welcome? As the church grew from a tiny seed into a small mustard bush, it was as God designed it, but over time, it mutated into a large tree, something never intended by God.
This plant ceased to be God's church when it perverted its doctrines and objectives, moving beyond God's intended limits. It became a counterfeit of the true church, appropriating the name "Christian" and blending or syncretizing pagan mystery religions with Christianity. Eventually, it called itself the Roman Catholic Church, and later produced Protestant daughter churches whose doctrines are rooted in Catholicism.
What became of the true church? When the mustard plant mutated from its original form, God replanted His true church in another corner of the field, beginning the process anew. It is a consistent characteristic of God's true church to remain as a small herb, spiritually feeding the few who are chosen to become regenerated children of the Kingdom of God.