There are three types of consciences that people have:

1. Some people have no strong basis for making godly decisions. Their conscience is weak, and they can be easily swayed by the behavior of others. Doubters, Those who depart from the faith or church bench warmers who follow wherever the wind of life blows them.

2.  Then there are those whose conscience is defiled. The standard by which they measure good and evil is corrupted, polluted, and impure. Titus 1:15-16 “Everything is pure to those whose hearts are pure. But nothing is pure to those who are corrupt and unbelieving, because their mind and consciences are corrupted. Such people claim they know God, but they deny him by the way they live. They are detestable and disobedient, worthless for doing anything good.” These are those who listen to anything that sounds good to the ear, giving heed to deceiving spirits and doctrines of demons.

3.  But saddest of all are those who have a “dry as bone” conscience. According to 1 Timothy 4:2, these kinds of people are hypocrites and lairs whose consciences are dead. They have resisted that inner voice for so long that they no longer hear what it has to say. Their conscience either cease to function or functions in wickedness, rendering judgments that oppose the truth of the Gospel. Non believers, Atheist, Homosexuals, fraudsters and Lairs.

How does a person get to the point where their conscience is seared or defiled?

1. By receiving spiritual lies
Satan and his demons do not normally confront their victims with error. Instead they work through people who speak lies about scriptural truth,through the false teaching of deceiving spirits and demons.

2. By Speaking out of an Evil Heart
(Mat 15:18-20) "But those things which proceed out of the mouth come from the heart, and they defile a man. "For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false witness, blasphemies. "These are the things which defile a man, but to eat with unwashed hands does not defile a man."

3. From Perverse Sexual Relations or other Abominations to God
Engaging in un-natural sexual relations like Homosexuality,Incest and sleeping with the underage. Carrying out acts of abomination to God like offering scrafices to idols.

4. From Unforgiven Sin leading to bitterness (and defilement)
Hebrew 12:15 states that the root of bitterness if allowed grows up in people, leading to the corruption of many. It is this seed of bitterness that has grown in some islamic countries around the world, corrupting many, leading to terrorist acts. What will make a human being go to the shopping mall with the sole aim to kill innocent people who have done him/her no personal wrong.

5. From Ungodly Words, Attitudes and Actions - even for believers.
2 Peter 2:7-8 showcases that even a righteous man like Lot in the midst of immorality had to be resued by God because his soul was being tormented.

6. From a Lack of Knowledge of truth
1 Cor 8:7-9 reminds us of those with weaker consciences who seek approval from God by what they eat. What they lack is the knowledge of truth. However we who proclaim that we know the truth with our so called "Superior Knowledge" must be careful not to cause those with a weaker conscience to stumble. That is why seeking the truth by reading the scriptures is key. Therein we get out freedom from spiritual bondage.

7. By worshipping or Acknowledging Idols.
(Ezek 23:30 ) 'I will do these things to you because you have gone as a harlot after the Gentiles, because you have become defiled by their idols.

The consequences of having a seared conscience.

People with a seared or defiled conscience are ready to do Satan's bidding. i.e,  When their consciences are rendered beyond feeling and they increasingly lose their sense of right and wrong. Sadly, people whose consciences have been seared, never have their sin forgiven, because:
They are unaware of their sin in the first place. They are not obedient to 1 John 1:9. So their sin remains unforgiven. The consequences of this unforgiven sin are:

1. The Lord does not hear them ( Psalm 66:18 )
2. They are not able to abide in Christ and bear fruit ( John 15:4 )
3. Show they do not love Christ ( John 14:21 )
4. They cannot expect to learn all things ( John 14:26 )
5. They cannot expect to be guided into all truth ( John 16:13 )
6. They cannot remain in Christ's love ( John 15:10 )
7. Their joy cannot remain full ( John 15:11 )
8. They cannot be completely set free from the law of sin and death. ( Romans 8:2)
9. Christ cannot give life to their mortal bodies ( Romans 8:11 )
10. They will remain carnally minded and live in death. (Romans 8:6 )
11.  They walk in darkness and cannot and do not practice the truth. (1 Jn 1:6 )
12. They cannot have proper fellowship. ( 1 john 1:7 )
13. They deceive themselves and the truth is not in them. ( 1John 1:8 )
14. They hate their brother and are blind. ( 1 John 2:11 )
15. They cannot love others for God does not abide in them. ( 1 John 4:12 )

But you ask, “How can we have a cleansed conscience?”

We must repent of our sin and return to our Creator.
We must ask Him to conform our desires and behavior to His Word and then be careful to obey it.

Let the word of God dwell in you richly so that you can be equipped to do every good work. You must work hard in everything you do so you can present yourself to God and receive his approval. Not approval of men. Abide in the word of God and teach these truths to other trustworthy people who will be able to pass them on to others. The christian life is not to promote individualism, but to be proclaimed to others. The purpose of this is to open the eyes of those who have not accepted Jesus as their Lord and Savior to turn from darkness to light and the power of satan to God. You have recieved forgiven of sins and a place among God's people who are set apart by faith in Jesus. All humans deserve this inheritance.

CONCLUSION
At the day of Judgement with a clear conscience we can proclaim as Apostle Paul in 2 Timothy 4:6-8
As for me, my life has already been poured out as an offering to God. The time of my death is near. I have fought the good fight, I have finished the race, and I have REMAINED FAITHFUL. And now the prize awaits me- the CROWN OF RIGHTEOUSNESS, which the Lord, the righteous Judge will give me on the DAY OF HIS RETURN . And the PRIZE is not just for me but for all who eagerly look forward to his appearing."