I say the truth in Christ—I am not lying; my conscience confirms it by the Holy Spirit. (Romans 9:1)
Our inner man has a voice. We call it the conscience. An unregenerate man has a conscience, but it's unreliable. It might have been trained by social norms, family customs, and the like, but it doesn't echo or fall in line with the Spirit of God. The Word makes it clear that the psychical man ("natural man" in the King James translation) cannot receive from the Spirit of God; indeed, his mind, his thoughts, aren't neutral. They're at enmity with God, and can't be reigned in to follow his Spirit.
And Paul, earnestly beholding the council, said, "Men and brethren, I have lived in good conscience before God until this day." (Acts 23:1, KJV)
Paul's testimony before the Jewish council and his letter to the Roman Christians at first blush appear curious, even contradictory. He had persecuted the church, trying to destroy it. He even oversaw the stoning of Stephen, the church's first martyr. How could he say that he had lived in good conscience before God?
It was because he had been born again on the road to Damascus.
So if anyone is in Christ, there is a new creation: everything old has passed away, see, everything has become new. (2 Corinthians 5:17)
When a man believes that Jesus was raised from the dead, and he confesses him as Lord, he is born again. His inward man is made new. Paul writes that he is a new creation, born after the image of Christ, and the things of the old man, including the old man's voice, his conscience, are passed away. He has the nature of God within him, and that nature will lead him to lend his body to performing fruits of righteousness for the kingdom.
Once Paul was born again, his inner man became a sure voice for following the Lord and his Spirit. He never violated what his conscience told him from the day he met the Lord on the road to Damascus forward. That is how he could say before the council that he had lived in good conscience before God.
What agreement has the temple of God with idols? For we are the temple of the living God; as God said, "I will live in them and walk among them, and I will be their God and they shall be my people." (2 Corinthians 6:16)
Like Paul, the reborn man's conscience will lead him down the path of righteousness. The voice of his conscience will bubble up into his soul, enlighten his mind, and inform all his decisions. It will influence who he is and what he does in the world. He will walk in step with the Lord's commandments, and be a witness of the gospel to the world at large.
Jesus answered him, "Those who love me will keep my Word, and my Father will love them, and we will come to them and make our home with them.” (John 14:23)