There is no salvation in the law but the result of accepting Christ is obeying the law.
If the law is the written code, i.e law of Moses, you are correct. However the word law referenced in Rom. 2:13 is referencing a law that was put into effect by angles after Jesus' crucifixion, resurrection and ascension. Note Heb. 7:12, Acts 7:53, Gal. 3:19.
There is no proviso in the written code that would allow for a resolution of the sin of murder which crucifying Jesus is. So that God could prove beyond a shadow of doubt that he is merciful, but only to those who have the faith to resolve the sin of Jesus' crucifixion to his satisfaction.