“Do we then make void the law through faith? God forbid: yea, we establish the law.”
In this passage, salvation by “faith” is being contrasted to salvation by “works” (Rom. 3:27), the works or “deeds” of the law (Rom. 3:28). The law demands 100% righteousness to be saved (Gal. 3:10; James 2:10,11). That means to be saved by the deeds of the law, you would have to bend the law to say that God will accept people who are only 75% righteous, or 88% righteous, or even 99% righteous.
But faith in the sacrifice of Christ for our sins doesn’t have to bend the law, it establishes the law. Faith acknowledges that “the law is holy, and…just, and good” (Rom 7:12), but that we are “carnal, sold under sin” (Rom 7:14). That is, faith establishes that there is nothing wrong with the law, there is something wrong with us. We can’t keep the law perfectly, so we must place our faith in the Christ who kept it perfectly for us, and then died a sacrificial death on our behalf.
It was because the righteousness of the law couldn’t be fulfilled by us that Christ “gave Himself for us” (Titus 2:14), that “the righteousness of the law might be fulfilled in us” by Him (Rom. 8:4). source