Teaching a believer that they can never fall, is the snare of the devil, it keeps believers in satans trap/snare.
John 10:29
29 My Father, who has given them to Me, is greater than all; and no one is able to snatch them out of My Father’s hand.
once you have learned the full will of God for your salvation, which is to be able as a mature man, to know the difference fully between good and evil:
Genesis 3
4 Then the serpent said to the woman, “You will not surely die. 5 For God knows that in the day you eat of it your eyes will be opened, and you will be like God, knowing good and evil.”