Jeremiah 17:10 (NIV)
I, the Lord, search the heart and examine the mind.
It's sheer arrogance on our part to think that we can somehow better ourselves through self-examination and self-critique. When gentiles like us walk in the light of our own consciences, however refined and sensitive they may be, we become a law unto ourselves (Romans 2:14). Our consciences may sometime condemn us while at other times they may affirm us. When we gentiles try to live pious lives like the Jews in the light of our conscience, it becomes our god and leader. When our consciences replace the Holy Spirit, we have set up our own standards and deify ourselves. The Holy Spirit searches our hearts and minds more intricately than our own conscience. A child of God should be led by the Spirit of God from the word of God. If something is affirmed in the bible, no matter how disgusting (ex: blood and flesh of Christ, or sex) it may be, then it's holy. Else, however noble and progressive it may be (ex: homosexuality), that's sinful. We have no business replacing the Holy Spirit with our own conscience.