Ash-Shaafi’ee, may Allaah have mercy on him, said: “Had Allaah not sent down aproof to His creation other than this surah (chapter), it would have been sufficient for them.”Al-Bukhaaree, may Allaah have mercy on him, said: “Chapter: Knowledge comesbefore speech and action.”The proof for this is Allaah’s saying:

So know that there is no deity worthy of worship except Allaah, and seekforgiveness for your sins.”

[Surah Muhammad: 19] So He began by mentioningknowledge before speech and action.
 

Know, may Allaah have mercy on you, that it is obligatory upon every Muslim maleand female to learn and act upon the following three things:
First:
That Allaah created us, provided sustenance for us, and did not leave usneglected. Rather, He sent a Messenger to us. So whoever obeys him will enter Paradise, whereas whoever disobeys him will enter the Hellfire.The proof for this is Allaah’s statement:
“Verily, We sent to you a Messenger, to be a witness over you, just as we sent aMessenger to Pharaoh. But Pharaoh disobeyed the Messenger (i.e. Moosaa) soWe seized him with a severe punishment”
[Surah Al-Muzzammil: 15-16]
Second:
Allaah is not pleased with anyone being mixed into worship with Him –neither an angel brought close nor a prophet that was sent.The proof for this is:
“And verily the
masaajid 
belong to Allaah, so do not call unto anyone along withAllaah.”
[Surah Al-Jinn: 18]
Third:
Whoever obeys the Messenger (i.e. Muhammad) and singles Allaah out (inworship), it is not permissible for him to take as allies those who oppose Allaah andHis Messenger, not even if they are close relatives.The proof for this is Allaah’s saying:
“You will not find any people who believe in Allaah and the Last Day makingfriendship with those who oppose Allaah and His Messenger, even if they aretheir fathers or their sons or their brothers or there kindred. For such He haswritten Faith in their hearts, and strengthened them with
Rooh
(light andguidance) from Himself. And He will admit them into Gardens under which riversflow to dwell therein forever. Allaah is pleased with them and they with Him.They are the party of Allaah. Verily it is the party of Allaah that will be thesuccessful.”

[Surah Al-Mujaadilah: 22]Know, may Allaah grant you the ability to obey Him, that Al-Haneefiyyah, the Religionof Ibraaheem, is that you worship Allaah alone, making the religion sincerely (i.e.solely) for Him. This is what Allaah ordered all of mankind to do, and this is thepurpose for which He created them, as Allaah says:

“And I did not create the Jinn and mankind except to worship Me.”
[Surah Adh-Dhaariyaat: 56]The meaning of 
“to worship Me”
is “to single Me out (in worship).”The greatest thing that Allaah commanded is Tawheed, which means: Singling Allaahout in worship. And the greatest thing that Allaah warned against is Shirk, which is:Supplicating to others along with Him.The proof for this is Allaah’s statement:
“And worship Allaah (alone) and do not mix anything (in worship) with Him.”
 [Surah An-Nisaa: 36]So if it said to you: What are the three fundamental principles that mankind is obligatedto know?Then say: That the servant knows his Lord, his Religion and his Prophet Muhammad,may the peace and blessings of Allaah be on him.