John Gill Commentary Romans 11:27

"And this is my covenant unto them, When I shall take away their sins." — Romans 11:27 (ASV)

For this is my covenant to them
This is what God has promised to them in covenant, and he will be as good as his word; his covenant will never be broken, it will always remain sure and inviolable; so that there is not only a possibility, and a probability, but even a certainty, of the call and conversion of the Jews; which promise and covenant will have their accomplishment,

when I ,
says the Lord,

shall take away their sins :
some think that the apostle alludes to (Jeremiah 31:34) ; others, that he takes this passage out of (Isaiah 27:9) ; where in the Septuagint version the selfsame phrase is used; though it may be no citation, or reference, but the apostle's own words, explaining what is meant by "turning away ungodliness from Jacob", (Romans 11:26) ; and as before; regards not the taking away of their sins by the sacrifice of Christ, which is done already, and is what the blood of bulls and goats could not do; but of the removing of their sins from themselves, from their consciences, by the application of the blood of Christ, and the imputation of his righteousness.