Romans 5:20

"The Law came in so that the transgression would increase; but where sin increased, grace abounded all the more".

"It (the Law) came alongside, says Paul, 'in order that the offence might abound'. The word 'offence' refers to Adam's offence. And that ... includes our offences also ... so when he says, 'that the offence might abound' - he is referring to the sin of Adam and to all the sins of men ever since. The Law has come in alongside in order that this sin, these actual sins and offences might abound - that is to say 'might increase' or 'be augmented'.

What then is the function of the Law according to the Apostle? It has been brought in alongside in order that the offence, the sins might abound".

Reference: Dr Martyn Lloyd-Jones - "Exposition of Chapter 5 - Assurance" - Banner of Truth, Edinburgh (1998) - (p285).