The Apostle Paul wrote about some who finally came to a have a reprobate mind, so what is a reprobate and is there a risk we can reprobate?
A reprobate is someone who is an unprincipled person. To the hyper-Calvinist, they are a sinner who is not of the elect and is predestined to damnation. The way the Bible uses the word reprobate in relation to humans is someone who is a depraved, unprincipled, or wicked person: maybe even a drunken reprobate, but a person rejected by God and beyond hope of salvation. The reprobate has no desire to repent because they love their sin; they’ve rejected godly principles and only live by the lust of the eyes, the lust of the flesh, and the pride of life. If you work with your hands, you’ll get callouses. These build up to protect your hand from injury, but callouses also cover some of the pain receptors or nerve endings in our hands. You can jab your hand accidentally over a calloused hand and cause less injury that one that has no callouses. The same thing can happen to our hearts. The more we “turn off” God in our lives and suppress the voice of the Holy Spirit, the harder it will be to hear from Him.
A Seared Conscience
The idea of a reprobate mind may start with the mind that becomes seared, or having a seared conscience. Continually living in sin has a way of making us grow comfortable with it, but there’s danger in that. We become desensitized to sin and see it as less and less of an issue, however the Apostle Paul warned that “the Spirit explicitly says that in later times some will fall away from the faith, paying attention to deceitful spirits and doctrines of demons, by means of the hypocrisy of liars seared in their own conscience as with a branding iron” (1 Tim4:1-2). If you put a branding iron to your hand (don’t do this!), it’ll burn the pain receptors away, and that part of the hand will not feel pain again (after it heals), so sin sears our conscience and takes away the conviction of sin. This is bad. This happened in ancient Israel where Jeremiah the Prophet asked the rhetorical question, “Were they ashamed when they committed abomination? No, they were not at all ashamed; they did not know how to blush. Therefore they shall fall among the fallen; when I punish them, they shall be overthrown, says the LORD” (Jer 8:12). They were so callous to sin that they had even lost the ability to blush (Jer 6:15). That means, nothing embarrassed them; nothing was too much for them; and everything was permissible because they had seared their conscience and no longer felt guilt or conviction of sin. Their conscience’s voice, after having been ignored and suppressed for years, was finally silenced once and for all.
A Good Conscience
James, the brother of Jesus said that “whoever knows the right thing to do and fails to do it, for him it is sin” (James 4:17). It is sin because we have the power or ability to help someone. The Apostle John said that “if anyone has the world’s goods and sees his brother in need, yet closes his heart against him, how does God’s love abide in him” (1st John 3:17)? Love is a verb; it’s what you do, infinitely more than what you say, so how come we so often turn our backs on those who need our help? Maybe we’re suppressing His voice. What would we do if we saw “a brother or sister is without clothes and daily food” (James 2:15)? Would we say, “Go in peace, be warmed and filled,’ without giving them the things needed for the body, what good is that” (James 2:16). Constantly turning away to help others in need, especially when we have it within us to help, won’t sear our conscience, but it’ll surely convict us. Or, at least it should.
A Reprobate Mind
Just because someone is not a believer, doesn’t mean they don’t know right from wrong. We’re all going to stand before Christ and given an account for our life, but all of us are without excuse for denying the existence of God (Psalm 19:1-2; Rom 1:18-20), and that all of us have fallen short of God’s glory (Rom 3:10-12, 23). We are all responsible for our choices. Today, as in the Apostle Paul’s day, “Even the “Gentiles who do not have the Law do instinctively the things of the Law, these, not having the Law, are a law to themselves, in that they show the work of the Law written in their hearts, their conscience bearing witness and their thoughts alternately accusing or else defending them, on the day when, according to my gospel, God will judge the secrets of men through Christ Jesus” (Rom 2:14-16). When their conscience bears witness to the reality that they are sinning, “they become a law to themselves,” so they become lawbreakers, and all lawbreakers have a band ending (Rev 20:12-15). The self-proclaimed “carnal Christian” should be careful that their sin doesn’t carry them away by slow and steady deception. Sin is sneaky…it has a way of overtaking us without our knowing it, “Therefore let anyone who thinks that he stands take heed lest he fall” (1 Cor 1)12). Pride comes first…then the fall (Prov 16:18).
The reprobate mind is no longer capable of feeling conviction over their sin, and so they give up their bodies to whatever works or feels best, so what happens to people like this? Paul says “since they did not see fit to acknowledge God, God gave them up to a debased mind to do what ought not to be done” (Rom 1:28), and I won’t go into all the things that “ought not to be done.” You probably have a good idea, but what works best for us is to repent at the moment we’ve sinned, and strive to “lay aside every weight, and sin which clings so closely, and let us run with endurance the race that is set before us” (Heb 12:1).
Here is some related reading for you: What is a Reprobate Mind? 
Resource – Scripture quotations are from The Holy Bible, English Standard Version® (ESV®), Crossway Bibles. (2007). ESV: Study Bible: English standard version. Wheaton, Ill: Crossway Bibles. Used by permission. All rights reserved.