May peace be with you.
With humble respect, this is a popular misconception. Hearts are only sealed
once complete manifestation of truth has been made to a soul according to their faculties and resources despite which it is
unduly rejected by the individual. Once one has rejected clear evidence,
only then is one’s case abandoned and fate sealed.
Only God has that capacity to judge.
I cordially invite you to read the article below which argues the definition of ‘Kufr’ from a Quran’s perspective.
I hope that helps, God willing.
Joseph.
REFERENCE[1] UNDERSTANDING 'KUFR' (DISBELIEF) FROM A QURANIC PERSPECTIVEhttp://quransmessage.com/articles/understanding%20kufr%20FM3.htm