Grafting is a horticultural technique of splicing a bud or branch of one tree into the trunk/root system of another. Over time, the splice heals, allowing the new branch to thrive and bear fruit—if such is its nature. Inherent in its nature, too, is that the branch continues to bear the type of fruit it originally produced, i.e., an apple branch grafted into an orange trunk (if possible) does not now bear oranges.
In his letter to the Romans, Paul uses grafting to illustrate the concept of the Gentile believers being included in the blessings of Abram:
"...if the root is holy, the branches are too. But if some of the branches were broken off, and you, being a wild olive, were grafted in among them and became partaker with them of the rich root of the olive tree," (Romans 11:16-17 NASB unless noted)
Although it is assumed that Paul's emphasis is not on the trunk of the olive tree but on the root and the branches, the inference above states that the cultivated branches removed represent unbelieving Israel—and the wild olive branches refer to Christian Gentiles, a.k.a. Gentile followers of Christ.
"You will say then, "Branches were broken off so that I might be grafted in. Quite right, they were broken off for their unbelief, but you stand by your faith." (Romans 11:19-20)
"And they [Israel] also, if they do not continue in their unbelief, will be grafted in, for God is able to graft them in again. For if you were cut off from what is by nature a wild olive tree, and were grafted contrary to nature into a cultivated olive tree, how much more will these who are the natural branches be grafted into their own olive tree?" (Romans 11:23-24)
From these verses, we can deduce that:
Since the cultivated branches that were cut off represent Israel, the trunk cannot then represent Israel. The trunk is not cut off, only the branches. The rich root, described as being holy or sacred, must represent Yeshua, the Messiah:
"Again Isaiah says, "THERE SHALL COME THE ROOT OF JESSE, AND HE WHO ARISES TO RULE OVER THE GENTILES, IN HIM SHALL THE GENTILES HOPE." (Romans 15:12)
The joining factor between the root and the branches, the trunk of the tree, grows out of the covenant with YHWH. Does the trunk then symbolize the covenant? I believe it does. The covenant is the basis by which the branches, Israel and, for now, the Christian Gentiles, enjoy a relationship with YHWH.
In Genesis 17, we read the details of YHWH's promise to Abraham:
"I will establish My covenant between Me and you and your descendants after you throughout their generations for an everlasting covenant, to be God to you and to your descendants after you. I will give to you and to your descendants after you, the land of your sojournings, all the land of Canaan, for an everlasting possession; and I will be their God." (Genesis 17:7-8)
Abram was a Gentile, an idol worshipper, married to his half-sister—major negatives to being a good Jew.
Four hundred and thirty years later, YHWH "cut" another covenant with all the children of Israel at Mount Sinai. This, a bilateral covenant, required every Israelite to adhere to the Law given through Moses, in addition to circumcision, as conditions for living in covenant with YHWH.
Those of the children of Israel could be cut off if they broke any part of that covenant.
"For whoever does any of these abominations, those persons who do so shall be cut off from among their people." (Leviticus 18:29)
This prospect is repeated throughout the Torah. The consequence of breaking the law is that of being "cut off" from among the people. Again, if Israelites are the branches that can be "cut off" from among the other branches, then Israel cannot be the tree; it must be the branches. This is inferred in Romans 11:23-26. Israel will be grafted in again.
The covenant is the binding factor joining the branches to the rich and holy root.
Paul writes that because faith is the sole basis through which the Gentile believers can be in relationship with YHWH, being united (grafted in) in the Messiah to become partakers of the blessing of Abraham—via the promises of Genesis 12—according to Paul.
"...in Christ Jesus the blessing of Abraham might come to the Gentiles, so that we would receive the promise of the Spirit through faith." (Gal 3:14)
I believe Paul is clear that the opportunity the Gentiles have in being grafted in was made possible because Israel broke the terms of their covenants and was "cut off.” The reference to the Abrahamic covenant is significant in that the requirements of that covenant are significantly less demanding than the Mosaic covenant, which includes things such as strict observance of the Sabbath, food laws, feast days, etc. All of which was obligatory on the sons of Israel but were not a part of the Abrahamic covenant.
When the fullness of the Gentiles has come in, Israel will again be grafted into the truck.
"a partial hardening has happened to Israel until the fullness of the Gentiles has come in and so all Israel will be saved" (Romans 11:25-26)
There remain some questions—do those Christian Gentiles who were grafted in need to be removed so that Israel can be grafted back in? Does this necessitate a rapture event?
The mystery hidden for ages past—the seed of the woman planted, dying to itself so that it could become the root that provides the living water to any or all who are grafted into the covenant.
"Oh, the depth of the riches both of the wisdom and knowledge of God! How unsearchable are His judgments and unfathomable His ways!" (Romans 11:33)