Because even if someone has received the heavenly calling, that person has received only an invitation, not a final confirmation of the reward.
لِأَنَّهُ حَتَّى لَوْ تَلَقَّى أَحَدُ ٱلْمَسِيحِيِّينَ ٱلدَّعْوَةَ ٱلسَّمَاوِيَّةَ ،‏ فَهٰذَا لَا يَعْنِي أَنَّ مُكَافَأَتَهُ تَأَكَّدَتْ بِصُورَةٍ نِهَائِيَّةٍ .‏