It should be clear that if a person misses salatul juma three or more times due to laziness then he will not be considered Kafir (Non Believer) however he has committed a great (Kabeerah) sin. As far as the statement that the heart is sealed is concerned, it means that the person will be prevented from doing blessed deeds
ALLAH Almighty knows best
DARULIFTA AL-IKHLAS,Karachi