Romans 11:19-27
Rom 11:19 (KJV)
Thou wilt say then, The branches were broken off, that I might be grafted in.

In verse 18 Paul told the Gentiles not to boast or glory against the branches and here he continues his warning to the Gentiles. They may say that the branches were broken off to allow the Gentiles to be grafted in to the body of Christ. Just as the smaller trees were grafted into the larger trees, so each true believing Gentile is grafted into the larger body of Christ.

Rom 11:20 (KJV)
Well; because of unbelief they were broken off, and thou standest by faith. Be not highminded, but fear:

Paul says “well” in that he agrees with the statement that the branches were broken off so the Gentiles might be grafted in. However, he goes on to say that they were broken off because of unbelief.
(Heb 4:6 KJV) Seeing therefore it remaineth that some must enter therein, and they to whom it was first preached entered not in because of unbelief:  The Gentiles were grafted in by faith. Faith is what Israel had lacked. The Gentiles here are being warned not to be or act arrogantly against Israel. Paul wanted them to know that arrogance was not the right attitude because if these people remain unsaved, then they will be going to Hell for eternity, so pity is what was needed to be showed and not arrogance as if the Gentiles were the ones who saved themselves. They were to live in fear, not of eternal damnation, but that they offend God and God brings a chastisement upon them.

Rom 11:21 (KJV)
For if God spared not the natural branches, take heed lest he also spare not thee.

God had brought judgments upon the nation of Israel which caused them to lose their spiritual position. Paul is warning anyone who thinks they are out of the realm of the chastising hand of God to beware.
(1 Cor 10:11 KJV) Now all these things happened unto them for ensamples: and they are written for our admonition, upon whom the ends of the world are come. What happened to Israel was to serve as an example for us. God does not tolerate arrogance in His church or in His people. If we think we can get away with anything evil, we do not understand the true nature of God.

Rom 11:22 (KJV)
Behold therefore the goodness and severity of God: on them which fell, severity; but toward thee, goodness, if thou continue in his goodness: otherwise thou also shalt be cut off.

The word “severity” in the Greek carries with it the meaning of “strict or cut.” Notice that Paul places “goodness” and “severity” of God as two opposite traits of God but are both definite traits of God. The nation of Israel, because of their unbelief, had experienced the strict nature of God concerning obedience. Those who became born again through the blood of the Lord Jesus Christ experienced the goodness of God in that the imputed righteousness of Christ was given to those Gentiles. It was as if, they had kept the law perfectly through the Lord Jesus Christ. In the last phrase of this verse it almost looks like if the Gentiles do not continue in the goodness of God, it seems like they can lose their salvation. This can never happen since God grants us eternal life. What is in view here is that God will begin pruning the branches of the tree. A corresponding principle to the above phrase is found in John 15:2:
(John 15:1-2 KJV) I am the true vine, and my Father is the husbandman. {2} Every branch in me that beareth not fruit he taketh away: and every branch that beareth fruit, he purgeth it, that it may bring forth more fruit. In this verse we see that if a Christian lives a fruitless, wanton life, God reserves the right to remove that branch. He will do so by physical death and that is how the arrogant Gentile can be cut off.

Rom 11:23 (KJV)
And they also, if they abide not still in unbelief, shall be grafted in: for God is able to graft them in again.

Those Israelites whom God will save will be grafted into the Body of Christ in the same manner as those Gentiles who have become saved. God is able to graft both Jew and Gentile into the Body of Christ, for there is no respect of persons with God.

Rom 11:24 (KJV)
For if thou wert cut out of the olive tree which is wild by nature, and wert grafted contrary to nature into a good olive tree: how much more shall these, which be the natural branches, be grafted into their own olive tree?

The unsaved Gentiles who were in opposition to God are able to become saved and grafted into the body of Christ which is totally opposite of the sinful ways of the world. The kingdom of Christ and the kingdom of Satan are two different kingdoms run on two different ideologies. God is able to take any unbeliever out of the kingdom of Satan and save them, and then transplant them into the kingdom of Christ. Then Paul states that the Israelites, who have had the teachings of God for 1,500 years, when they become saved, it is like being grafted back into their own natural environment, under the teaching of God. The Gentiles did not have the teaching of God and the Hebrew Scriptures which Israel did. The Gentiles had to learn the ways of the Lord and that is why Paul had his hands full with the churches since everything was new to the saved Gentiles.

Rom 11:25 (KJV)
For I would not, brethren, that ye should be ignorant of this mystery, lest ye should be wise in your own conceits; that blindness in part is happened to Israel, until the fulness of the Gentiles be come in.

Paul now gives them a revealed mystery so they will not think of themselves as wiser than others. Israel was spiritually blinded, only in part, because there were many who were becoming saved, as God had promised that a remnant out of Israel would become saved. This blindness had opened the door for the Gentiles to come into the Kingdom of God. Paul states that this blindness not only allowed the Gentiles to come in, but they would come in until the last day, when the last one is saved who is to become saved.

Rom 11:26 (KJV)
And so all Israel shall be saved: as it is written, There shall come out of Sion the Deliverer, and shall turn away ungodliness from Jacob:

The word “so” in the Greek carries with it the meaning of “in this manner” or “in this way.” Paul is stating that in this manner, all Israel shall be saved. What manner is that? Now many have erroneously interpreted this verse to mean that sometime in the future all of national Israel is going to be saved. This is a very fallacious interpretation because God is not giving salvation based on a race. The Israel that is going to be saved, is the entire body of Christ and not a physical nation. The body of Christ is an amalgamation of saved Gentiles and Jews. This is how the entire body of Christ is going to be saved. Below are some verses which teach that the Israel in view, is the body of Christ and not a physical nation.

(Gal 6:16 KJV) And as many as walk according to this rule, peace be on them, and mercy, and upon the Israel of God.

(Phil 3:3 KJV) For we are the circumcision, which worship God in the spirit, and rejoice in Christ Jesus, and have no confidence in the flesh.

(Rom 2:28-29 KJV) For he is not a Jew, which is one outwardly; neither is that circumcision, which is outward in the flesh: {29} But he is a Jew, which is one inwardly; and circumcision is that of the heart, in the spirit, and not in the letter; whose praise is not of men, but of God.

(Rom 9:6 KJV) Not as though the word of God hath taken none effect. For they are not all Israel, which are of Israel:

(Gal 3:29 KJV) And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise.

(Eph 2:12 KJV) That at that time ye were without Christ, being aliens from the commonwealth of Israel, and strangers from the covenants of promise, having no hope, and without God in the world:

(Isa 45:17 KJV) But Israel shall be saved in the LORD with an everlasting salvation: ye shall not be ashamed nor confounded world without end.

The verse then goes on to say that the Deliverer will come out of Zion and drive ungodliness from Jacob.
(Isa 59:20 KJV) And the Redeemer shall come to Zion, and unto them that turn from transgression in Jacob, saith the LORD. The Deliverer is the Lord Jesus Christ, also known as the Redeemer in Isaiah 59:20. Jacob is another term for the body of Christ because only those whom the Lord saves shall have ungodliness turned away in their lives. In other words, the Redeemer makes them clean. Zion is another term for the body of believers. (Psa 99:2 KJV) The LORD is great in Zion; and he is high above all the people.

Rom 11:27 (KJV)
For this is my covenant unto them, when I shall take away their sins.

Here Paul brings in the Abrahamic Covenant. The eternal covenant is the only one which saves people and removes their sin. God had made a covenant with the entire house of Israel that He was going to take away their sins. The Israel in view is not the physical nation of Israel but the body of Christ.
(Jer 31:34 KJV) And they shall teach no more every man his neighbour, and every man his brother, saying, Know the LORD: for they shall all know me, from the least of them unto the greatest of them, saith the LORD: for I will forgive their iniquity, and I will remember their sin no more. (Heb 8:12 KJV) For I will be merciful to their unrighteousness, and their sins and their iniquities will I remember no more. It is the body of Christ which has had their sins removed and not the physical nation of Israel.