And even as they did not like to retain the Lord in their knowledge, the Lord gave them over to a reprobate mind, to do those things which are improper; Rom 1:28 Men 'did not like to retain God in their knowledge'.     This means that they once knew there was a God but chose to disavow Him because they did not like Him.      Every human being is born with a sense that there is a Supreme Being.      On what basis does a person decide to reject the notion of God?      Rebellion .      Every other explanation they offer is just cover for the fact that they hate God and wish that He did not exist.     Such are children of Satan - the first rebel.      This is why repentance is needed to come to God in Christ Jesus.     One must see themselves as a rebel in opposition to the very One who created them and loved them enough to die for their reconciliation to Himself.      Those who persist in their rebellion against God spiral down eventually getting to the point where they are unreachable by the Truth.      Such end up with Reprobate Minds because they knowingly and willfully reject the Truth, Jesus Christ.      There are two major results of this:    ➛ they do things which are self-destructive    ➛ the Holy Spirit ceases to draw them.

The most telling characteristic of a person with Reprobate Mind is that they do not consider God in their knowledge or thinking.   

This leads them to say and do things that are "not convenient".   
"Not convenient" means "not normal", "not common-sensical", "not sane".   

Being filled with all unrighteousness, fornication, wickedness, covetousness, maliciousness; full of envy, murder, debate, deceit, malignity; whisperers, Rom 1:29 Those people who death spiral as noted in Rom 1:21 are then filled with the works of the flesh which is Satanically inspired.

Backbiters, haters of the Lord, despiteful, proud, boasters, inventors of evil things, disobedient to parents, Rom 1:30 When a person hates the Lord Jesus Christ they will hate everyone who loves the Lord, speaks of Him or reminds them of Him.      Fundamentally, they become despisers of auhority and so make themselves their own god.     This was Lucifer`s sin and it is the one he communicates to humanity.

Without understanding, covenantbreakers, without natural affection, implacable, unmerciful: Rom 1:31 It is important to note that a person who reaches this state will be `without understanding`.     The Common Sense blessing from the Lord will be completely missing in them.     Their affection will be affected as opposed to genuine and it will be directed to things which are unholy.

Who knowing the judgment of the Lord, that they which commit such things are worthy of death, not only do the same, but have pleasure in them that do them. Rom 1:32 Christians appreciate the support of likeminded believers but they do not depend on them.     Whereas unbelievers need their support group because of the guilt that they can`t escape.

From the above we see that their disconnection from God results in all manner of sin.   The extent of their depravity is that they enjoy the fact that others participate in those sins.   

The reality is that the decision to deny God leads inexorably to a Reprobate Mind.   Disconnection from the knowledge of God has negative short-term, long-term and eternal consequences

Heb 11:6 says that a person cannot come to God without believing that He is .   Psm 14:1 says that a person who denies God`s existence is a fool.   

So, God does not have to do anything to a person for them to have a Reprobate Mind.   They do it to themselves.   It is an act of their will.   God respects their decision and gives them over to their desire.   

No one can make such a statement without having rejected the Truth.   

The Word is Truth Joh 17:17
Jesus is Truth Joh 14:6
Holy Spirit is the Spirit Of Truth Joh 16:13

Do not waste time with such.   Simply tell them that they will come to know the Truth when they bow before Him in judgment just before being cast into the Lake Of Fire Rev 20:15 .   

If a person asks you how you know the Bible is true they are seeking Truth.   Such are open to the Truth.   Witness to them of your experience in Christ.   Share with them the fulfilled prophecies - especially those regarding the Lord Jesus Christ.