Great discussion!
What my understanding of it is, simply put, is a person who is beyond allowing the Holy Spirit to use them. Their desires and agendas override the will of God and scripture. That's why it's dangerous for preachers to become preachers for egotistical reasons which is why so many church's have the issues they do today.
A reprobated mind will promote their philosophy's of life over scripture. Many of them will twist scripture to conform to what they want it to mean rather than rightly dividing the word. A reprobate will not seek the understanding they need to continue to receive wisdom, knowledge and understanding from the Holy Spirit.
They come to a point where they "know it all" and can't nobody tell them anything...which will ultimately make them the biggest fools around because the Word of God and the Lord is not revealed completely in any one person's life time. People come close. But the truest Bible scholars will even say that every day the Bible is revealing something new. A reprobated mind is not open to the newness, therefore they stay stuck in old patterns, and they'll stay stuck for life because they don't believe they need to change. Truly believe they understand the Bible, when they don't and it ends up being to their detriment.
Many of them think they can Hallelulah, sing and dance their way to heaven and nothing else. Many reprobates come from people who were born and raised up in the ministry. There are some pastors kids who love saying "Oh I didn't grow up in the world" They believe because their parents were who they were then they have this automatic entrance into heaven. I call them modern day Pharissees and Sadducees. Biggest heathens up in church continuing backwards Christianity through generations, twisting the word to fit whatever they like putting on shows behind the pulpit, yelling and screaming and not saying a Word from the Holy Spirit.
Everyone must be careful, but for the most part those who are called to be used, and I do believe every Christian is used in someway shape or form, you don't have to be preaching. Just keep in mind there is never a point in life where you can just push the cruise button on your salvation. We must constantly study, constantly search ourselves. Reprobates will study and search everybody and their mama before themselves.