God gives each one of us a conscience. If we let the Holy Spirit take over, God will show us clearly what's good and what's evil, and we shall all think as God does. What happens sadly today is that because people follow their own wishes and desires, their conscience has become so muddy that they no longer can distinguish between the good and the evil. Take unmarried couples living together. I know for a fact that one couple is doing it because they can claim that one parent has low income so that the kids can get the government to pay for the college tuition. Generally, couples do it because lots of other couples are doing it. How about abortion? If you don't think about it, "to have the freedom of choice" just sounds good. So you believe that abortion is not really wrong, but deep down you know that it's not quite right either. If it's totally right, then why isn't there much joy and peace after an abortion? The conscience is "gone" because people are trapped in sin, because everybody is doing it, because it's easy to turn away from conscience, and because they don't bother to do any deep soul searching.
On the other hand, when you let the light of Christ shine upon you, all sins are exposed and with repentance and God's forgiveness, the conscience is restored to its pristine state. The sharp conscience then keeps you living in a state of grace, with peace and joy.