John Calvin Commentary Romans 11:12

"Now if their fall, is the riches of the world, and their loss the riches of the Gentiles; how much more their fulness?" — Romans 11:12 (ASV)

And if their fall, etc. As the apostle had taught that after the Jews were repudiated, the Gentiles were introduced in their place, he anticipates this false notion (lest the salvation of the Jews be disliked by the Gentiles, as if their salvation depended on the ruin of the Jews) and presents an opposing principle: that nothing would contribute more to advance the salvation of the Gentiles than for the grace of God to flourish and abound among the Jews.

To prove this, he derives an argument from the less — “If their fall had raised the Gentiles, and their diminution had enriched them, how much more their fullness?” For the first was done contrary to nature, and the last will be done according to a natural order of things.

And it is no objection to this reasoning that the word of God flowed to the Gentiles after the Jews had rejected it and, as it were, cast it from them. For if they had received it, their faith would have produced much more fruit than their unbelief caused. Indeed, the truth of God would have been thereby confirmed by being fulfilled in them, and they themselves also would have led many by their teaching—many whom, on the contrary, they had turned aside by their perverseness.

Now, he would have spoken more precisely if, to the fall, he had opposed rising: I remind you of this so that no one may expect elaborate language here, nor be offended by this simple way of speaking, for these things were written to mold the heart and not the tongue.