Allaah Jalla Jalaalahoo is that Being whose Knowledge encompasses the outward and inward, the open and the hidden, the necessary, probable and the impossible, the upper and the lower, the past, present and future. Allaah Jalla Jalaalahoo has Always Known and will Always Know what has happened and what will happen.  Nothing in the heavens and earth is hidden from Him.

His Knowledge encompasses all things, their external and internal, their intricacies, and that which is apparent in the most perfect manner. Allaah Jalla Jalaalahoo Knows whatever the hearts of His Creation conceal i.e., imaan, kufr, truth, falsehood, good, evil, and those things which people want to conceal in their hearts which as yet they have not as yet thought of. Allaah Jalla Jalaalahoo has mentioned :

اِنَّ اللهَ لَا يَخْفى عَلَيْهِ شَىْءٌ فِى الْاَرْضِ ولَا فِى السَّمَآءِ

“Verily, nothing in the earth and in the heavens is hidden away from Allaah.”
(Aal-Imraan)

In this verse, the All-Encompassing Knowledge of Allaah Jalla Jalaalahoo is mentioned, showing that no atom in the universe is concealed from Him.
In Surah An’aam, extra detail has been mentioned:

 

وَهُوَ اللّهُ فِي السَّمَاوَاتِ وَفِي الأَرْضِ يَعْلَمُ سِرَّكُمْ وَجَهرَكُمْ وَيَعْلَمُ مَا تَكْسِبُونَ َ

“And He is Allaah, in the heavens and earth (i.e., He is the Only Deity, Master and Sustainer of the whole universe). He Knows that which you hide, and that which you make apparent, and He Knows that which you earn.”

When His Rule Extends over the heavens and earth, and without any intermediary, He is Fully Aware of every concealed and open thing, the inward and outward, the major and minor actions of every person, then the worshipper is not in need of attributing any partner with Allaah Jalla Jalaalahoo in His Worship, and in seeking help from Him.

The following supplication makes manifest the quality of Allaah Jalla Jalaalahoo being Al-Aleem. We should continuously recite this duaa, and we should also continuously read its translation with contemplation. 

 

يَامَنْ لَاتَرَاهُ الْعُيُونُ، وَلَاتُخَالِطْهُ الظُّنُونُ، وَلَايَصِفُهُ الْوَاصِفُونَ، وَلَاتُغَيِّرُهُ الْحَوَادِثُ، وَلَايَخْشَى الدَّوَائِرَ، يَعْلَمُ مَثَاقِيلَ الْجِبَالِ، وَمَكَايِيلَ الْبِحَارِ، وَعَدَدَ قَطْرِالْأَمْطَارِ، وَعَدَدَ وَرَقِ الْأَشْجَارِ، وَعَدَدَ مَاأَظْلَمَ عَلَيْهِ اللَّيْلُ، وَأَشْرَقَ عَلَيْهِ النَّهَارُ، لَاتُوَارِي مِنْهُ سَمَاءٌ سَمَاءً، وَلَا أَرْضٌ أَرْضًا، وَ لَا بَحْرٌ مَا فِي قَعْرِهِ، وَ لَا جَبَلٌ مَا فِي وَعْرِهِ، اجْعَلْ خَيْرَ عُمُرِي آخِرَهُ، وَ خَيْرَ عَمَلِي خَوَاتِمَهُ، وَ خَيْرَ أَيَّامِى يَوْمَ أَلْقَاكَ فِيهِ

“O that Being Who eyes cannot see, Who minds cannot fathom, Who praises cannot fully praise, Who the vicissitudes of time cannot change, and He Who does not fear the passing of time, the One Who Knows the weight of mountains, the measurement of oceans, the number of raindrops, the number of leaves on the trees, the number of all those things upon which the night darkens, and upon which the day shines, that Being from Whom one heaven cannot conceal another heaven, nor one earth another earth, that Being from Whom the ocean cannot conceal what is in its depths, and the mountains cannot conceal what it in its caves; make the best portion of my life, my last portion, and my best action, my final one, and my best day, the day in which I meet You.”
(Mu’jamul-Awsat of Tabraani)

 

The differences between Allaah Jalla Jalaalahoo’s Knowledge and man’s knowledge:

1) Allaah Jalla Jalaalahoo’s Knowledge is One, in which every matter is known equally; whereas man’s knowledge is different, since man is in need of different types of knowledge with regards to different things.

2) Allaah Jalla Jalaalahoo’s Knowledge never changes, whilst man’s knowledge is in a constant state of flux.

3) Allaah Jalla Jalaalahoo’s Knowledge is intrinsic, and not acquired; whereas the knowledge of man is acquired through the senses.

4) It is impossible for Allaah Jalla Jalaalahoo’s Knowledge to terminate, whereas man’s knowledge can terminate.

5) Various types of Knowledge of Allaah Jalla Jalaalahoo cannot prevent Him from having other knowledge; whereas this is not the case with man.

6) Allaah Jalla Jalaalahoo’s Knowledge is Infinite, whereas man’s knowledge is finite and limited.