The Reward of those Who fear their Lord unseen
إِنَّ الَّذِينَ يَخْشَوْنَ رَبَّهُم بِالْغَيْبِ لَهُم مَّغْفِرَةٌ وَأَجْرٌ كَبِيرٌ
Verily, those who fear their Lord unseen, theirs will be forgiveness and a great reward.
Allah informs of he who fears standing before his Lord, being frightened about matters between himself and Allah when he is not in the presence of other people. So he refrains from disobedience and he performs acts of obedience when no one sees him except Allah.
Allah mentions that this person will have forgiveness and a great reward. This means that his sins will be remitted and he will be rewarded abundantly.
This is similar to what has been confirmed in the Two Sahihs,
سَبْعَةٌ يُظِلُّهُمُ اللهُ تَعَالى فِي ظِلِّ عَرْشِهِ يَوْمَ لَا ظِلَّ إِلاَّ ظِلُّه
There are seven people whom Allah the Exalted will shade in the shade of His Throne on the Day when there will be no shade except its shade.
Then he mentioned that among those people are;
دَعَتْهُ امْرَأَةٌ ذَاتُ مَنْصِبٍ وَجَمَالٍ فَقَالَ إِنِّي أَخَافُ اللهَ
وَرَجُلً تَصَدَّقَ بِصَدَقَةٍ فَأَخْفَاهَا حَتْى لَا تَعْلَمَ شِمَالُهُ مَا تُنْفِقُ يَمِينُه
- A man who is tempted by a beautiful woman of high social status, but he says;`Verily, I fear Allah.'
- Another person from among them is a man who gives charity and he conceals it so that his left hand does not know what his right hand spent.
Then He says, while informing that He is aware of the innermost conscience and secrets,
وَأَسِرُّوا قَوْلَكُمْ أَوِ اجْهَرُوا بِهِ إِنَّهُ عَلِيمٌ بِذَاتِ الصُّدُورِ