[Biblestudent]

I'll try to explain:
Romans 11:32 For God hath concluded them all in unbelief, that he might have mercy upon all.

ROMANS 1 - Gentiles' UNBELIEF
ROMANS 2 - Jews' UNBELIEF
ROMANS 3 - All have SINNED

1. The Gentiles served the creature more than the Creator (Rom. 1).
So God concluded all the Gentiles in unbelief, waiting for God's wrath.

2. Israel had the law but disobeyed it (Rom. 2)
So God concluded all Israel in unbelief, waiting for God's judgment.

[Romans 9:4 Who are Israelites; to whom pertaineth the adoption, and the glory, and the covenants, and the giving of the law, and the service of God, and the promises;

Ephesians 2:11 Wherefore remember, that ye being in time past Gentiles in the flesh, who are called Uncircumcision by that which is called the Circumcision in the flesh made by hands;
Ephesians 2:12 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: ]

Hebrews 10:30 For we know him that hath said, Vengeance belongeth unto me, I will recompense, saith the Lord. And again, The Lord shall judge his people.

3. So God concluded them all - both Jews and Gentiles - in unbelief that he may have MERCY upon all.

Romans 3:9 What then? are we better than they? No, in no wise: for we have before proved both Jews and Gentiles, that they are all under sin;

So the explanation for Romans 11:32 can be found in the context:

Romans 11:25 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.
Romans 11:26 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:
Romans 11:27 For this is my covenant unto them, when I shall take away their sins.
Romans 11:28 As concerning the gospel, they are enemies for your sakes: but as touching the election, they are beloved for the fathers' sakes.
Romans 11:29 For the gifts and calling of God are without repentance.
Romans 11:30 For as ye in times past have not believed God, yet have now obtained mercy through their unbelief:[Gentiles received mercy due to Israel's unbelief.]
Romans 11:31 Even so have these also now not believed, that through your mercy they also may obtain mercy. [Now, the Jews may also receive mercy through the mercy received by Gentiles.]
Romans 11:32 For God hath concluded them all in unbelief, that he might have mercy upon all.