The following verse must refer to all believers, both Jews and Gentiles:
"For Christ is the end of law for righteousness to every one that believes" (Ro.10:4; DBY)).
Since this is speaking about "every one that believes" it must be referring to "law" (which includes both The Law and the law written in the heart of which the conscience bears witnessed. This translation of the same verse could not be correct since it is limited to "The Law":
"For Christ is the end of the law for righteousness to everyone who believes"
(Ro.10:4; ESV).
No, they were under the law written in their hearts of which the conscience bears witness:
"Wherefore by works of law no flesh shall be justified before him; for by law is knowledge of sin" (Ro.3:20; DBY).
Before we are aware of the Ten Commandments we know what constitutes sin because our conscience tells us that certian things are wrong.