With the knowledge of good and evil man came into the possession of conscience. A sense of blameworthiness smote him when he committed, or even contemplated committing, evil. This has been so ever since. The Bible tells us that even the most ungodly and benighted heathen “show the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the meanwhile accusing or else excusing one another” (Rom. 2:15).
It is true that man’s conscience can be violated so often that it becomes calloused or, as St. Paul puts it: “seared with a hot iron” (I Tim. 4:2), but events or incidents can take place which suddenly awaken the conscience and make it sensitive again. Many a person has indulged in “the pleasures of sin” more and more freely until, suddenly, his sin has found him out and his conscience has caught up with him to condemn him day and night and make life itself unbearable.
The Bible teaches that all men outside of Christ are, to some degree, troubled by guilty consciences and certainly most are “through fear of death… all their lifetime subject to bondage” (Heb. 2:15). But it also teaches that “Christ died for our sins” so that our penalty having been paid, we might be delivered from a guilty conscience.
The works and ceremonies of the Mosaic Law could never accomplish this, but sincere and intelligent believers in Christ, having been “once purged”, have “no more conscience of sins” (Heb. 9:14; 10:1,2). They are, to be sure, conscious of their sins, but they are no longer tortured by a forever-condemning conscience, for they know that the penalty for all their sins, from the cradle to the coffin, was fully met by Christ at Calvary.
This is not to imply that even a sincere believer may not be troubled about offending the One who paid for his sins, but he knows that the judgment for these sins is past. Thus he earnestly seeks, like Paul, “to have always a conscience void of offence toward God and toward man” (Acts 24:16). source