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 [Rom. 11:25].

“The fulness of the Gentiles” began with the calling out of the church. “Simeon hath declared how God at the first did visit the Gentiles, to take out of them a people for his name” (Acts 15:14). It will continue until the rapture of the church. Blindness and hardening of Israel will continue as long as the church is present in the world.
The word mystery needs a word of explanation. In the ancient world of Paul’s day there were mystery religions. Today it applies in a popular way to a story that has an unrevealed plot or person. It is used in Scripture in neither of these ways. In the New Testament the word is used to refer to that which had been concealed but is now revealed. The mystery here is the identification of the fullness of the Gentiles, which was not a subject of revelation in the Old Testament.

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:

For this is my covenant unto them, when I shall take away their sins [Rom. 11:26–27].

When Paul says “all Israel shall be saved,” he does not mean every individual Israelite will be saved. It is the nation he has before us in this chapter. In every age, only a remnant is saved. The quotation Paul uses is from Isaiah 59:20 in the Old Testament: “And the Redeemer shall come to Zion, and unto them that turn from transgression in Jacob, saith the LORD.” The message to the individual is that he will have to “turn from transgression” to the Lord. There will be a remnant that will turn to Him. All of them will be saved. He speaks of the saved remnant as the nation Israel.
There is always only a remnant that is saved. There was a remnant in Elijah’s day; there was a remnant in David’s day; there was a remnant in Paul’s day; there is a remnant in our day; and there will be a remnant during the Great Tribulation period.

As concerning the gospel, they are enemies for your sakes: but as touching the election, they are beloved for the fathers’ sakes.

For the gifts and calling of God are without repentance [Rom. 11:28–29].

In other words, with reference to the gospel, they are enemies for your sakes; but with reference to the election, they are beloved for the sake of the fathers. For the gifts of grace and the calling of God are without repentance—without a change of mind. Paul is summing up the preceding discussion. There have been two lines of thought which are seemingly in conflict and contradictory, although both are true. In the first place, Israel is regarded as an enemy for the sake of the Gentiles—that is, so the gospel can go to the Gentiles. On the other hand, they are beloved for the sake of Abraham, Isaac, and Jacob. Therefore, a Christian cannot indulge in any form of anti–Semitism—that is a point I have made before, and continue to make it.
The failure of Israel and our failure likewise do not alter the plan and purpose of God.
“The gifts” are not natural gifts, but the word has to do with grace.
The “calling” is not an invitation, but it is the effectual calling of God, which is “without repentance.” In other words, God is not asking even repentance from an unsaved person. The “calling of God” does not require any human movement. From God’s viewpoint it is without man’s repentance or change of mind. Some folk think they have to shed tears in order to be saved. Now certainly the shedding of tears could be a by–product of an emotional person who turns to Christ, but the tears have nothing in the world to do with your salvation. It is your faith in Christ that saves you. And neither is your faith meritorious. It is Christ who is meritorious. Your faith enables you to lay hold of Him.

For as ye in times past have not believed God, yet have now obtained mercy through their unbelief:

Even so have these also now not believed, that through your mercy they also may obtain mercy [Rom. 11:30–31].

You see, Paul is writing to Gentiles—the church in Rome was largely composed of Gentile believers. By this time, many Gentiles were being saved. He is drawing a contrast here between the nation of Israel and the Gentiles. In times past, the Gentiles did not believe, but now a remnant of the Gentiles have “obtained mercy.” During this same time period Israel as a nation, which formerly believed, does not now believe. Paul puts down the principle by which God saves both Jew and Gentile: it is by mercy. Just as God showed mercy to the Gentiles, He will show mercy to the nation Israel.

For God hath concluded them all in unbelief, that he might have mercy upon all [Rom. 11:32].

Both Jew and Gentile are in the stubborn state of rebellion and aggravated unbelief. Because of this, by grace we are saved, through faith; and that not of ourselves, it is the gift of God; not of works, lest any of us should boast (see Eph. 2:8–9].

REASON FOR RESTORING THE NATION ISRAEL

What is the reason that the nation Israel will be restored? Well, that is locked in the riches of the wisdom of God. My friend, let’s rest on the fact that what God is doing is wise, it is right, and it is the best that can be done. You and I have an old nature that questions God when He makes a decision. I have heard many Christians say, “Why are the heathen lost when they haven’t heard the gospel? God has no right to condemn them!” My friend, God has every right imaginable. He is God. And what He is doing is right. If you don’t think it is right, your thinking is wrong. And if you don’t think He is being smart, you are wrong. God is not stupid. You and I may be stupid, but God is not. Oh, how we need to recognize this!

O the depth of the riches both of the wisdom and knowledge of God! how unsearchable are his judgments, and his ways past finding out! [Rom. 11:33].

Paul has come to the place of recognizing the wisdom and the glory of all that he has been discussing.
Godet’s statement on this section is worth quoting: “Like a traveller who has reached the summit of an Alpine ascent, the apostle turns and contemplates. Depths are at his feet, but waves of light illumine them, and there spreads all around an immense horizon which his eye commands.”
This section is pure praise and is no argument at all, yet it is the greatest argument of all. If we do not understand the why of God’s dealings with Israel, with the Gentiles, and with ourselves, it is not because there is not a good and sufficient reason. The difficulty is with our inability to comprehend the wisdom and ways of God. “But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned” (1 Cor. 2:14).
Once, while driving back from Texas to California, my little girl developed a fever of 104 degrees. I took her to a hospital in Phoenix, Arizona. She did not understand why I had taken her to the hospital, especially when the doctor probed around and actually made her cry. She said, “Daddy, why did you bring me here?” She did not understand that, since she was sick, I was doing the wisest thing I could do under the circumstances and that I was doing it because I loved her. Oh, my friend, God is doing what is best for us. We may not understand the things that happen to us, but we must believe that it is for our good that God allows them. We are like little children, and we cannot understand God’s ways. Our circumstances may not always seem to be good, but they come from the “depth of the riches both of the wisdom and knowledge of God.” God says to us, “For my thoughts are not your thoughts, neither are your ways my ways, saith the LORD. For as the heavens are higher than the earth, so are my ways higher than your ways, and my thoughts than your thoughts” (Isa. 55:8–9). Oh, how we need to recognize this fact.

For who hath known the mind of the Lord? or who hath been his counsellor?

Or who hath first given to him, and it shall be recompensed unto him again? [Rom. 11:34–35].

These questions that we have here are simple enough, but the answer is not so easy.
“Who hath known the mind of the Lord?” Well, no one knows the mind of the Lord—that’s the answer. It was Paul’s ambition to know Him. He says, “That I may know him, and the power of his resurrection, and the fellowship of his sufferings, being made conformable unto his death” (Phil. 3:10).
“Who hath been his counsellor?” No one can advise God. I have seen a lot of church boards that felt they were really giving God good advice, but He doesn’t need it. Have you noticed that the Lord Jesus never asked for advice when He was here on earth? One time—before feeding the five thousand—He asked Philip, “… Whence shall we buy bread, that these may eat?” Why did He ask that question? “And this he said to prove him: for he himself knew what he would do” (John 6:5–6). He didn’t need Philip’s advice. The fact of the matter is, he didn’t use His disciples’ advice. They said, “Send them away.” He said, “You give them something to eat.” My friend, God does not ask for advice, although a lot of folk want to give Him advice today.
“Who hath first given to him?” Have you ever really given anything to God which put Him in the awkward position of owing you something? If you were able to give God something, He would owe you something. What do you have that He hasn’t already given you? I think one reason many of us are so poor is simply because we return to Him so little of what He has given us. To tell the truth, God says He won’t be in debt to anybody. When somebody gives Him something, He turns around and gives him more. Years ago someone asked a financier in Philadelphia, a wonderful Christian man, “How is it that you have such wealth, and yet you give away so much?” The financier replied, “Well, I shovel it out, and God shovels it in; and God’s shovel is bigger than my shovel!” Oh, my friend, most of us are not giving God a chance to use His shovel! We cannot do anything for Him—He will give us back more than we give to Him.

For of him, and through him, and to him, are all things: to whom be glory for ever. Amen [Rom. 11:36].

This just lifts me to the heights. Let me give you my translation: Because out of Him, and through Him, and unto Him are all things. To Him be the glory unto the ages. Amen.
Alford labeled this verse “the sublimest apostrophe existing even in the pages of inspiration itself.”
“Out of Him” means God is the all–sufficient cause and source of everything.
“Through Him” means God is the mighty sustainer and worker. “… My Father worketh hitherto, and I work,” Jesus said (John 5:17).
“Unto Him” means God must call every creature to account to Him. All things flow toward God.
“To whom be glory”—the glory belongs to Him in all ages. Are we robbing God of His glory by taking credit for things we have no business to claim? The glory belongs to Him.
Oh, my friend, what a section of Scripture we have been in, and we leave it reluctantly."  McGee, J. V. (1991). Thru the Bible commentary: The Epistles (Romans 9-16) (electronic ed., Vol. 43, pp. 60–65). Nashville: Thomas Nelson.