A branch may be cut off and separated from the vine - but no branch shall ever be separated from Christ, this heavenly Vine. Some tell us that a justified person may fall away finally. Is not Christ a perfect Vine? He is not perfect if a living branch may be plucked off from Him. Has not Christ said of His elect, "I give them eternal life, and they will never perish—ever! No one will snatch them out of My hand!" John 10:28. If any branch is plucked away from Christ, it is either because Christ is not able to keep it—or because He is willing to lose it. He is surely able to keep it, for He is strengthened with the Godhead; and He is not willing to lose it, for why, then, would He have shed His blood for it? So that no branch shall ever be separated from the celestial Vine. You may sooner pluck a star out of the sky—than a true believer from Christ. Indeed, hypocrites who look like branches, fall off—but they were never really in the Vine. They were in Christ by profession, not by union. They were tied on to the vine but not engrafted. An elect branch can no more perish than the root! (p. 405, A Plea for the Godly, Watson, Soli Deo Gloria)There was no union in truth for those who profess faith and ultimately perish apart from the people of God. No truth can be plainer than this, and Watson nails this doctrine. The vinedressers of the church can tie a graft (a new professor of Christ) to the trunk of the vine - and water the vine, care for it as for the other branches...we, being Gentiles, are those wild branches that are, at the outset, tied to the trunk. That is the step of a new professor being brought to the church and making profession, joining the body in that sense.
Now if the sap of Christ's blood doesn't flow into that graft, it withers and dies, and is cast aside. No life comes to that branch unless it comes from the root, through the trunk and into the branch itself. If that connection is there, if the sap flows, then life comes to the branch and fruit flourishes. However, if that vital connection fails to be made, then there is no hope, and the branch will fail and there is no hope for it. Any vinter knows that if a grape vine is tied, in an attempt to graft it to the living vine, and there is no union of cells that apply the pressure in the root to drive sap up into the grafted-on branch, then they'll never see grapes produced on that graft.
Why any maintain based on John 15 that there is true union with Christ for any but the elect is beyond my ability to understand.