Matthew Poole and continuators on Galatians 2:21

21 I do not frustrate the grace of God , for if righteousness come by the law , then is Christ dead in vain .

I do not despise, reject, make void (for by all these words the word here used is Translated; Mark 7.9. Iohn 12.48. chap. 3.15. Heb. 10.28.) the free love of God, in giving his Son to dye for our sins: From whence is easily gathered, that those who live a loose life, and take a liberty to sin from their Justification, or from the free grace of God in Christ; they do contemn and despise the Grace of God: Or rather, (if we refer it to the following words) those who assert Justification by the works of the Law, they do reject and despise the free grace of God in the Gospel, and (as much as in them lyes) make it vain and frustrate.

For if it be possible, that a man by works done in obedience to the Law, should arrive at a Righteousness, in which he may stand before God,

there Christ dyed to no purpose, or without any just cause; the reason of this must be, because it was the main and principal end of Christs death, to procure or purchase a Righteousness, wherein sinners might stand before God, to bring in an everlasting Righteousness▪ Dan. 9.27. If the most proper effect of the death of Christ be taken away, then his death is made causless, and to no purpose. Thus the Apostle concludeth his Thesis, laid down verse 16. That none shall be justified by the works of the Law, from two absurdities that would follow upon the contrary, viz. Iustification by the works of the Law, the rejection of the grace of God, and the frustration, or making void of the death of Christ.

Marginal references: Heb. 7.11.