Do we then overthrow the law by this faith? By no means! On the contrary, we uphold the law.
31. Do we then make, etc. When the law is opposed to faith, the flesh immediately suspects that there is some contrariety, as though the one were adverse to the other: and this false notion prevails, especially among those who are imbued with wrong ideas as to the law, and leaving the promises, seek nothing else through it but the righteousness of works.
From all this Paul infers that it is in vain to look for justification by the works of the law, and that it is to be had only by faith, which is the point he has been all along proving, from Rom 1:17, and which he lays down (Rom 3:28) as the summary of his discourse, with a quod erat demonstrandum - which was to be demonstrated.
Commenting on Romans 3:19-31
Verse 31. Do we then make void the law. Do we render it vain and useless; do we destroy its moral obligation; and do we prevent obedience to it, by the doctrine of justification by faith ? This was an objection which would naturally be made; and which has thousands of times been since made, that the doctrine of justification by faith tends to licentiousness.