Now its important to realize that forgiveness from God is the deciding factor, as to if a persons forgiven. This may seem obvious, however. It is what it is.
"so that while seeing, they may see and not perceive, and while hearing, they may hear and not understand, otherwise they might return and be forgiven.” Mark 4:12 NASB
This concept God continually mentioned in the old testament (Ref: Isaiah 6:9, Isaiah 43:8, Jeremiah 5:21, Jeremiah 36:3, Ezekiel 12:2) All visible from the bible hub page.
Yet it was through the Jeremiah reference that he defines the condition of forgiveness.
"Perhaps the house of Judah will hear all the calamity which I plan to bring on them, in order that every man will turn from his evil way; then I will forgive their iniquity and their sin.” (Jeremiah 36:3)
Notice he says "will hear". Jesus also says "He who has ears to hear, let him hear." two times. Once in Mark 4:9 and once in Mark 4:23.
In Mark 4:24 He states, "Take care what you listen to."
Followed by a "how you measure what you hear statement".
In Mark 4:20 He states, "they hear the word" and makes reference to those that do hear as the "good ground".
Therefore by the statement that he made: "Do you not understand this parable? How will you understand all the parables?" in Mark 4:13
Since this parable specifically was about a person hearing, this gives us direction about "all the rest of his parables."
And when all of his parables become understood. Then indeed, a person will understand what Jesus was saying. Then they can believe him.