Romans 4:15

because the law brings wrath. And where there is no law there is no transgression.

(v15, NIV)

So, the law and faith is contrasted by Paul. Clearly he has and continues to be promoting faith and saying that Abraham's faith was credited to him as righteousness. Now talking about the promise, it makes sense for it to be by faith and not based upon the law. The thing is that the law brings wrath. In addition to this, transgression is in there where there is the law. Paul describes this point by making the opposite assertion. He says that where there is no law, there is no transgression. Since there is transgression, there is wrath. Wrath is brought upon by the breaking of the law. Instead of bringing righteousness and salvation through the promise based on faith and God's faithfulness, the law brings the wrath of God for the transgressing of his law.