Two things. Our very own conscience, and the accuser of the brethren. Our conscience because God wrote His laws upon our hearts, even unbelievers have a conscience. All humanity has a conscience, but of course people go against their conscience. Some would argue people have them seared and thats why there is no standard of morality for them, it is all subjective. People have an inherent sense of right and wrong, though people (who dont believe the bible) would associate that with some type of social evolutionary reason (whats best for survival).
Now on to the second answer, the accuser of the brethren. Well, the devil doesn't want you to know who you are in Christ and to have confidence towards God. So he attacks with condemnation and guilt so as to deter you from the Lord's presence. The veil is broken but he, as with legalists, would have it sown back up. Let me show you a verse about the disarming of principalities and powers.
Colossians 2:14-15New King James Version (NKJV)
14 having wiped out the handwriting of requirements that was against us, which was contrary to us. And He has taken it out of the way, having nailed it to the cross. 15 Having disarmed principalities and powers, He made a public spectacle of them, triumphing over them in it.
Notice that the disarming of the enemy was due to the Law being nailed to the cross? So as the accuser of the brethren, what can't he do now? He can't point to our falling short of the glory of God, and condemn us, distancing us from God as Jesus fulfilled the Law and has imputed us with His righteousness. We don't let our hearts condemn us and we can have confidence towards God because of this gift of no condemnation. The enemy may show us our shortcomings but we rest in the grace of God and the finished work of Jesus, knowing fully we may come boldly into His presence.
This site does a little examination of some of the words in the text to make a point in the greek:
Colossians 2:15 - Andrew Wommack Ministries