What is the meaning of Sealed Heart that reads in many of the Quranic Verses? How one who has a sealed heart could be held responsible for not believing?

Sealed Heart is defined as hearts getting blacken and stiffen because of unbelief and repentance.

Messenger of Allah (peace be upon him) stated, With every other sin that one commits, heart gets one more black spot.

In a verse, Assuredly God does not forgive that partners be associated with Him; less than that He forgives to whomever He wills (whomever He has guided to repentance and righteousness either out of His pure grace or as a result of the persons choosing repentance and righteousness by his free will). Whoever associates any partner with God indeed fabricated a most heinous sin. (An-Nisaa Surah, 4:48) As we understood from the hadith and the verse, associating any partner with God is the main cause for heart to become blacken. If one becomes the advocate of the Shirk (polytheism) and starts to fight against believers, the black spot enlarges and darkens more and more; finally covers the whole heart so it becomes impossible for him to accept the belief and tawheed (unification). With the expression of the Risale-i Nur collection, author Bediuzzaman Said Nursi it is, It corrupts mans potentialities in such a way that they are incapable of reform and unreceptive to good.

Above quoted verse has been announced about the unbelievers who fight against the Messenger of Allah (pbuh). The door, which opens to the way of Allah (swt), is closed for unbelievers who reached up to that level of disbelief. It does not fit to all sinners, oppressors or even to people who associate partners with God. If it had been, how could we explain the people who later became Muslims but used to be idolaters in Asr-i Saadah (Age of Happiness and tranquility in which Prophet Muhammad lived).

If anyone associates partners with God are set sealed in their hearts, no unbeliever should not have become Muslims. As a matter of fact, persons who have sealed hearts are the ones who cannot comprehend the Unification (Oneness of God).

And these people fall into a hole by misusing their will power.

We would like to take your attention to an important point. In the Risale-i Nur Collection Disbelief is analyzed in two grounds: Non-acceptance and acceptance of non-being. Non-acceptance is confused with the acceptance of non-being. The evidence for non-acceptance is the absence of established proof. The acceptance of non-being requires proof of non-being. One is doubt while the other one is denial. One is unconcernedness, closing eyes to the facts, and ignorance. However, in the acceptance of non-being, there is an advocacy, and invitation of disbelief. As you see second group fights against believers. Sealed heart is more effective for them actually. We say more, because even it is rare there are some people who find the way of Allah and become Muslims.