I won't be able to summarise it adequately, so I apologize in advance. There's isn't enough space to write a detailed summary so this will just be skimming the surface.
I'll begin with my failure to understand Genesis 3 which speaks about Adam's fall, the introduction of sin into the world, and the consequential enslavement of humanity to sin. The "Fall of Man" and "Original Sin," where Adam, acting as the covenant representative (or federal head) of all humanity, disobeyed God, resulting in the corruption of human nature and the alienation of mankind from God.
I was of the view that our inherited corrupt human nature didn't prevent us from choosing to turn from our sin to repent, and place our trust in the Lord Jesus Christ for the forgiveness of our sin to receive salvation.
I have since learned that, because of the fall, man is unable of himself to savingly believe the gospel. The natural man (unregenerated) is dead, blind and deaf to the things of God. He is not free to choose, because he is enslaved to his evil nature so he can't choose good over evil in the spiritual realm.
I used to believe that the bible doctrine of "election" was based on God looking into the future to see how we respond to His gospel call. Then He only chose those who He knew would of themselves freely believe by their own faith in the gospel. So I believed that election was determined by the faith that man already had by his will.
I obviously don't believe that anymore, now I believe salvation is all of the Lord, He is the Author and the Finisher of my salvation.
I don't believe man has a "free will", because every part of our constitution has been corrupted. Our soul, mind, heart, and body have all been corrupted. we don't have the means by which we can restore our being to a state of sinless righteous state. It's like saying a bad tree can produce good fruit, it just can't happen unless God intervenes.
I used to believe that God would save the whole world, as in John 3:16 "for God so loved the world that he gave His only begotten Son, so whosever believes on Him shall not perish but have everlasting life". But there were major red flags with that idea, namely all of those passages of scripture that declare that the majority are not going to be saved. So, God either contradicts Himself or my understanding of that doctrine was wrong.
I used to believe that man ultimately had the choice to either believe or reject, the gospel. I have since come to believe that God regenerates those he chose to save, and He changes their nature and gives them the gift of faith, so they will gladly believe.
I used to believe that a born again believer, can die spiritually again and lose their salvation. I have since come to believe that God keeps His people saved forever. If salvation was left in our hands, the Devil would snatch it from us at some point. That's why God has to hold us in His hand, so nothing can snatch us out of His hand.
I would have liked to list many verses of scripture to support these views but, I'll need to do that in a future post