Conscience

I say the truth in Christ, I lie not, my conscience also bearing me witness in the Holy Ghost, Romans 9:1 (KJV)

Jesus is the truth:
Jesus saith unto him, I am the way, the truth, and the life: no man cometh unto the Father, but by me. John 14:6 (KJV)
When we are in Christ, Who is “The Truth,” we should speak the truth, live the truth, and never, never, never lie. Satan is the father of lies and has nothing to do with the truth.
Ye are of your father the devil, and the lusts of your father ye will do. He was a murderer from the beginning, and abode not in the truth, because there is no truth in him. When he speaketh a lie, he speaketh of his own: for he is a liar, and the father of it. John 8:44 (KJV)
The Christian must be a person whose word is trustworthy; a person of integrity. We are what we speak. Believers speak faith words, true words, honest, just, pure, lovely, virtuous words that bring a good report and are praiseworthy (Phil. 4:8 KJV)..
We having the same spirit of faith, according as it is written, I believed, and therefore have I spoken; we also believe, and therefore speak; 2 Corinthians 4:13 (KJV)
The Spirit-filled believer has a witness within that leads into all truth and warns against all lies.
Howbeit when he, the Spirit of truth, is come, he will guide you into all truth: . . . John 16:13 (KJV)
The Holy Spirit commingled together with your spirit speaks to you through your regenerate conscience. The conscience of the believer is a direct pipeline from the spirit of man to the conscious mind of man. Wherein it was unreliable in its natural state, it is now a reliable source of prompting and information from God by the Holy Spirit to your spirit. Your spirit communicates to your conscious mind by means of your conscience and thereby guards and keeps your mind.