"That is because they believed," meaning: they affirmed [the faith] with their tongues in public, "then they disbelieved," meaning: in their hearts, "so a seal was set upon their hearts," a seal was placed upon them so that they would not believe.
﴿ذَلِك بِأَنَّهُم آمنُوا﴾ يَعْنِي: أقرُّوا بألْسنتهم فِي الْعَلَانِيَة ﴿ثمَّ كفرُوا﴾ أَي: بقلوبهم ﴿فَطُبِعَ عَلَى قُلُوبِهِمْ﴾ ختم عَلَيْهَا أَلا يُؤمنُوا.
