In the context of Christianity, being grafted into the fig tree refers to the metaphor used in Romans 11:17, where Gentiles are compared to wild olive branches that are grafted into the cultivated olive tree. This metaphor signifies that, despite the rejection of Israel, God's grace has allowed Gentiles to share in the blessings and promises of the covenant established with Abraham.
There needs to be consistency when interpreting the New Testament. Things that are brought out in the New Testament as doctrine for the Church must have a corresponding element in the Old Testament. In other words, the New Testament is a spiritual revelation of the things consistent with the "Letter" of Old Testament precedent. There can be no New Testament reality unless there is first Old Testament precedent. In the Old Testament the prophets identify Israel as a "great congregation" and this is true. This great congregation of people came out of Egypt at the time of Moses and the earthly Tabernacle. The very definition of the Greek (language) "church" (ekklesia) means "called out [ones]" and this ties into Israel being "calling out" of Egypt by God and also refers to God "calling out" His Son from "out of" Egypt.
15 And was there until the death of Herod: that it might be fulfilled which was spoken of the Lord by the prophet, saying,
Out ["ek"] of Egypt have I
called ["kaleo"] my son. Matthew 2:15. (or, "ek-kaleo"]
And here:
8 Also at the same time Solomon kept the feast seven days, and all Israel with him, a very
great congregation, from the entering in of Hamath unto the river of Egypt. 2 Chronicles 7: 8.
Being consistent means to say the same thing as God. God called out the children of Israel out of Egypt; and God called out His Son from out of Egypt. Being consistent is to say in the New Testament the same thing we find in the Old Testament. If the animal sacrifice under the Law was slain to [temporarily] cover for one year the sins of the children of Israel, then this must also be the intent and purpose of the sacrifice of the Lord Jesus Christ on His cross at Calvary, that is, Jesus died to atone forever and finally the sins of the children of Israel the same as under the Law the animal was slain and sacrificed to cover and atone - whether for one year or not - the sins of the children of Israel.
This is what we call 'substitutionary sacrifice' which from the Garden was taught by God to His people (Gen. 1:12.)
To cover Adam and Eve from the elements of the now cursed ground God slay an animal and used its hide as clothing for these two persons in the Garden and out of the Garden. And before God sent these two out of the Garden He provided covering for their exposed flesh. This also bespeaks of our covering by God as opposed to our attempts to cover ourselves. Is it God's righteous acts towards man, or man's acts towards himself?
So, it is with the whole of the New Testament which is a record of the approximately first 30 years of the 'baby' Church of "called out [ones]" (Acts 1-28) which is a record of the spiritual Church of the great congregation of Hebrew people as first identified in the Old Testament as the children of Israel.
All the New Testament is, is the Mosaic Covenant between God and Israel revealed in the obedience of Jesus who died under the Law to atone for those under the Law, that is, Israel. The animal sacrificed under the Law as instructed and commanded by God for the high priest is the same that Jesus, the lamb OF GOD sent to die a substitutionary death for the children of Israel. Saul as rabbi and Pharisee understood this.
4 But when the fulness of the time was come, God sent forth his Son, made of a woman,
made under the law,
5
To redeem them that were under the law, that we might receive the adoption of sons. Galatians 4:4–5.
Nothing in the New Testament 'changed' the Old Testament. Rather, it is the New Testament record which is the fulfillment - NOT CHANGE - of the Old Testament truth. The animal sacrificed under the Law was used as instructed by God to atone yearly the sins of the children of Israel the same that Jesus died under the Law to atone for the sins of the children of Israel. Adding non-Hebrew Gentiles to Israel's atonement under the Law violates the truth found in the Old Testament. Jesus' death on His cross was used by God to atone finally and forever the sins of the children of Israel ONLY. For throughout Scripture the animal sacrificed every Yom Kippur in Israel was for the sins of the children of Israel, so, too, is Christ's death on the cross used by God to atone the sins of the children of Israel under the Law.