(by Jan Willem van der Houven)

Paul writes in Romans 11:25-26:
For I do not desire, brethren, that you should be ignorant of this mystery, lest you should be wise in your own opinion, that blindness in part has happened to Israel until the fullness of the Gentiles has come in. And so all Israel will be saved, as it is written: "The Deliverer will come out of Zion, and He will turn away ungodliness from Jacob"
Many Bible teachers and pastors read this verse with the view that they have already, wrongly, accepted: that there are two trees instead of one, two peoples of God - the church and Israel - and not one people, or ecclesia, of God.
For "the fullness of the Gentiles" in this verse they therefore interpolate the (Gentile-based) church. And they go even further. Having wrongfully interpolated 'the church' for "the fullness of the Gentiles", they then teach that this verse holds that the salvation of all Israel will only become possible after the church has come to her fullness and completeness, and is thus raptured away. Only then, they have been taught, will "all Israel be saved."
However this is NOT what this verse teaches.. What it says is that when the full number of Gentiles has come in, so all Israel will be saved, thus making the completing of the number of the Gentiles a part of all Israel.
The New Testament scriptures clearly speak that:

the two trees will become one tree

the two flocks will become one flock and that

the two sticks will become one stick.

Gentile believers, taken out of their wild olive tree, have already become part of the natural olive tree.
There are not two distinct peoples of God - the church on one side (soon to be raptured) and Israel on the other side (to go through another great tribulation) - as is wrongfully taught in so many circles.
We are living in the time period that the two flocks - the fullness of the Gentile believers and Judah (or modern Israel) will increasingly become one flock, one tree and one people of God as Jesus prophetically foretold when He said:
And other sheep I have which are not of this fold; them also I must bring, and they will hear My voice; and there will be one flock and one shepherd.  (John 10:16)
Or as it is written by Ezekiel:
Say to them, 'Thus says the Lord God: "Surely I will take the stick of Joseph, which is in the hand of Ephraim, and the tribes of Israel, his companions; and I will join them with it, with the stick of Judah, and make them one stick, and they will be one in My hand." ' And the sticks on which you write will be in your hand before their eyes. Then say to them, 'Thus says the Lord God: "Surely I will take the children of Israel from among the nations, wherever they have gone, and will gather them from every side and bring them into their own land; and I will make them one nation in the land, on the mountains of Israel; and one king shall be king over them all; they shall no longer be two nations, nor shall they ever be divided into two kingdoms again.  (Ezekiel 37:19-22)
This is a very important point and will certainly revolutionize the way any believer, from wherever he is among the nations, will relate to the soon-coming miracle, when the Holy Spirit will be poured out over all of Israel.
Why is this so revolutionary in our relationship with Judah, which is modern Israel?
Because if we are not a separate unit apart from Israel, but have been grafted in and made part of Israel, then there will not be a separate escape by way of a pre-tribulation rapture for just the Gentile grafted-in branches within the tree and commonwealth of Israel. Instead, we shall be here together until the final trumpet has sounded and we all will be meeting and welcoming the Lord in the air.
In the meantime, just as Zechariah (and also other prophets) foresees, the land of Israel and Zion will be the SAFEST country to dwell in during the days of the final tribulation. Thus soon will many believers, "ten out of each nation," find their future dwelling and safety by emigrating to Israel. This is how Zechariah spells it out:
The inhabitants of one city shall go to another, saying, "Let us continue to go and pray before the Lord, and seek the Lord of hosts. I myself will go also." Yes, many peoples and strong nations shall come to seek the Lord of hosts in Jerusalem, and to pray before the Lord. Thus says the Lord of hosts: 'In those days ten men from every language of the nations shall grasp the sleeve of a Jewish man, saying, "Let us go with you, for we have heard that God is with you." '  (Zechariah 8:21-23)
Other verses spell out the same truth:
For the Lord will have mercy on Jacob, and will still choose Israel, and settle them in their own land. The strangers will be joined with them, and they will cling to the house of Jacob.  (Isaiah 14:1)
Surely you shall call a nation you do not know, and nations who do not know you shall run to you, because of the Lord your God, and the Holy One of Israel; for He has glorified you.  (Isaiah 55:5)
The Gentiles shall come to your light, and kings to the brightness of your rising. 
The sons of foreigners shall build up your walls, and their kings shall minister to you; for in My wrath I struck you, but in My favor I have had mercy on you. Therefore your gates shall be open continually; they shall not be shut day or night, that men may bring to you the wealth of the Gentiles, and their kings in procession. For the nation and kingdom which will not serve you shall perish, and those nations shall be utterly ruined.
Also the sons of those who afflicted you shall come bowing to you, and all those who despised you shall fall prostrate at the soles of your feet; and they shall call you The City of the Lord, Zion of the Holy One of Israel.
Whereas you have been forsaken and hated, so that no one went through you, I will make you an eternal excellence, a joy of many generations. You shall drink the milk of the Gentiles, and milk the breast of kings; you shall know that I, the Lord, am your Savior and your Redeemer, the Mighty One of Jacob.  (Isaiah 60: 3, 10-12, 14-16)
One day soon, therefore, when the Holy Spirit falls upon all Israel  - from Dan to Beersheba as Ezekiel prophesies (see Chapter 37) - then the two flocks will become one, and more and more Gentile believers, understanding that they are, according to Paul (see Ephesians 2:10), part and parcel of the commonwealth of Israel, will come up to the land and people of Judah - or modern Israel - because they will realize that they are grafted into the tree of Israel.
This is what all these verses somehow proclaim. As we see the increasing darkness and anti-Semitic and anti-believers spirit that is already now taking over many nations, it is good to know that this scriptural possibility is there for those Gentiles believers who have joined themselves to the God and Messiah of Jacob.
"But on Mount Zion there shall be deliverance" (Obadiah 17) for as Isaiah prophesies:
For behold, the darkness shall cover the earth, and deep darkness the people; but the Lord will arise over you (Israel), and His glory will be seen upon you. The Gentiles shall come to your light.  (Isaiah 60:2-3a)
This indeed we shall see happen soon!