If we study the parable of the Mustard Tree we see that it relates to common OT themes.
Daniel 4,Ezekiel 31&17 show us that Christ is speaking of a type of Christian nation gone bad.Since the birds spoken of in Ezekiel 31 Matthew 13 Revelation 18 seem to relate to demonic principalities that have strongholds in the places of worship in Egypt,Babylon(NT & OT) & Assyria.We see the current state of our churches in the U.S.
In the Torah Mustard is not kosher in vedgetable gardens because as a seed bearing plant it is only permitted to be grown as a field plant yet Christ(a strict Jew)plants it in his garden.
This speaks of the fact that the people who made up the initial Mustard Seed were a very small group of Gentile believers who were made righteous(kosher) by faith.They were grafted on to Abraham (made his sons by faith).
This small group of faithful people were none other than the seperatists(Pilgrims) who landed on Plymouth Rock in 1620.They were the ones who Christ brought to himself then planted in his garden.