Romans 5:20

Moreover the law--"The law, however." The Jew might say, If the whole purposes of God towards men center in Adam and Christ, where does "the law" come in, and what was the use of it? Answer: It entered--But the word expresses an important idea besides "entering." It signifies, "entered incidentally," or "parenthetically." (In Gal 2:4 the same word is rendered, "came in privily.") The meaning is, that the promulgation of the law at Sinai was no primary or essential feature of the divine plan, but it was "added" (Gal 3:19) for a subordinate purpose--the more fully to reveal the evil occasioned by Adam, and the need and glory of the remedy by Christ. that the offence might abound--or, "be multiplied." But what offense? Throughout all this section "the offense" (four times repeated besides here) has one definite meaning, namely, "the one first offense of Adam"; and this, in our judgment, is its meaning here also: "All our multitudinous breaches of the law are nothing but that one first offense, lodged mysteriously in the bosom of every child of Adam as an offending principal, and multiplying itself into myriads of particular offenses in the life of each." What was one act of disobedience in the head has been converted into a vital and virulent principle of disobedience in all the members of the human family, whose every act of wilful rebellion proclaims itself the child of the original transgression. But where sin abounded--or, "was multiplied.

— Jamieson-Fausset-Brown Commentary (public domain)