In the name of Allaah, the Most-Merciful, the Bestower of Mercy.
Know, may Allaah have mercy upon you, that it is obligatory upon you to have knowledge of four matters:
- Knowledge (al-'Ilm), which is knowledge and awareness of Allaah, and knowledge of His Prophet, and knowledge of the religion of Islaam with the proofs.
- Action upon that.
- Calling to that.
- Patiently persevering and bearing any harm encountered upon that way.
The proof is the saying of Allaaah, the Most High:
"By time, mankind is in loss, except those who believe and worship Allaah alone, and do righteous good deeds, performing that which is obligatory upon them and avoiding that which they are forbidden, and enjoin one another with patient perseverance upon obedience to Allaah and in facing harm and trials." [Soorah al-'Asr (103)]
Ash-Shaafi'ee, may Allaah, the Most High, have mercy upon him said:
"If Allaah had sent down to His creation no other proof besides this Soorah, it would have been sufficient for them." 
Al-Bukhaaree may Allaah have mercy upon him, said:
"Chapter: Knowledge precedes speech and action" and the proof is the saying of Allaah, the Most High:"Know that none has the right to be worshipped except Allaah, and ask forgiveness of your Lord for your sins." [Soorah Muhammad (47):19] 
So He began by mentioning knowledge before speech or action.
Know, may Allaah have mercy upon you, that it is obligatory on every Muslim, male and female, to learn and act upon the following three matters:
The First: That Allaah created us and provided sustenance for us, and He did not leave us without a purpose rather He sent Messengers to us. Whoever obeys him will enter Paradise, and whoever rejects and disobeys him will enter the Fire, and the proof is the Saying of the Most High:
"We sent a Messenger to you, O people, as a witness in favour of you or against you on the Day of Resurrection, just as We sent a Messenger to the Pharaoh. But the Pharaoh denied and rejected the Messenger, so We seized him with a severe punishment." [Soorah al-Muzzammil (73):15-16]
The Second: Allaah is not pleased that anyone should be made a sharer in worship along with Him, neither any angel brought near, nor any prophet sent as a messenger, and the proof is the Saying of Allaah, the Most High,
"And the places of worship are for Allaah alone, so do not invoke anyone along with Allaah." [Soorah al-Jinn (72):18]
The Third: That whoever is obedient to the Messenger and singles out Allaah with all worship, upon tawheed, then it is not permissible for him to have friendship and alliance with those who oppose Allaah and His Messenger, even if they are those most closely related to him, and the proof is the Saying of Allaah, the Most High:
You will not find a people who believe in Allaah and the Last Day loving those who oppose Allaah and His Messenger; even if they are their fathers, or their sons, or their brothers, or theirkinsfolk. Rather Allaah has decreed true Faith for their hearts, and strengthened them with proof, light and guidance from Him; and He will enter them into the gardens of Paradise beneath whose trees rivers flow, and they will dwell therein forever. Allaah is pleased with them and they with Him. They are the Party of Allaah. Indeed the party of Allaah are the successful." [Soorah al-Mujaadilah (58):22]
Know, may Allaah direct you to obedience to Him, that the true and straight Religion, the way of Ibraaheem is that you worship Allaah alone making the Religion purely and solely for Him. This is what Allaah commanded all of the people with, and it was for this that He created them. Allaah, the Most High, says:
"I did not create jinn and mankind except that they should worship Me." [Soorah adh-Dhaariyaat (51):56]
And the meaning of worship ('ibaadah) here is to single Allaah out with all worship (tawheed). And the greatest of all that Allaah has commanded is tawheed which is to single out Allaah with all worship. The most serious thing that He forbade is shirk, which is to invoke others besides Him, along with Him. The proof is His Saying, the Most High:
"Worship Allaah alone, making all worship purely for Him, and do not associate anything in worship along with Him." [Soorah an-Nisaa (4):36]
So if it said to you, "What are the three principles which a person must know?" then say, "The servants knowledge of His Lord and His Religion (Deen), and his Prophet Muhammad (sal-Allaahu 'alayhe wa sallam).
 Tafseerul Qur’aanil 'Adtheem (8/500).
 Saheehul Bukhaaree (1/92).