Shaykh Muhammad ibn Saalih al-’Uthaymeen
The Shaykh said in Sharh Usoolul-Eemaan (pp.4-7):
 The deen (religion) of Islaam: It is the way of life that Allaah sent the Prophet Muhammad sallallaahu ‘alayhi wa sallam with. By it, Allaah terminated the validity of all other religions, perfected this religion for His worshippers, completed His favour upon them and has chosen only this religion for them – no other religion will be accepted by Him from anyone. Allaah – the Most High – said:
“Muhammad is not the father of any man amongst you. Rather he is the Messenger of Allaah and the khaatim (last and final) of the Prophets.” [Soorah al-Ahzaab 33:40].
“This day I have perfected your religion for you, completed My favour upon you, and have chosen for you Islaam as your religion.” [Soorah al-Maa'idah 5:3].
“Indeed the religion with Allaah is Islaam.” [Soorah Aal-'Imraan 3:19].
“Whosoever seeks a religion other than Islaam, never will it be accepted from him, and in the Hereafter he will be one of the losers.” [Soorah Aal-'Imraan 3:85].
And Allaah – the Most High – obligated all of mankind to take Islaam as their religion. So Allaah said, whilst addressing His Messenger sallallaahu ‘alayhi wa sallam:
“Say: O Mankind! Indeed I have been sent to you all as the Messenger of Allaah; to Whom belongs the dominion of the heavens and the earth. None has the right to be worshipped except Him; it is He who gives life and cause death. So believe in Allaah and His Messenger – the Prophet who can neither read nor write – who believes in Allaah and His Words. So follow the Messenger of Allaah so that you may be rightly-guided.” [Soorah al-A'raaf 7:158].
And in Saheeh Muslim (1/93), from Abu Hurayrah radiallaahu ‘anhu that Allaah’s Messenger sallallaahu ‘alayhi wa sallam said: “By Him in whose Hand is the life of Muhammad! There is no one from this nation, be he a Jew or a Christian, who hears of me and dies whilst not having eemaan (faith) in what I have been sent with, except that he will be one of the companions of the Hellfire.”
And eemaan (faith) in the Prophet implies: affirming that which he was sent with, along with acceptance of it and submission to it. Without these two matters, mere affirmation is not sufficient. This is why even though Abu Taalib (the Prophet’s uncle) affirmed what was sent to the Prophet sallallaahu ‘alayhi wa sallam and that Islaam was the best of religions, yet he did not accept his message nor submit to it; and thus he did not have eemaan in the Prophet sallallaahu ‘alayhi wa sallam.
 The religion of Islaam contains all that was beneficial from the previous religions. It is suitable for implementation in any age, any where and by any nation. Allaah – the Most High – said, whilst addressing His Messenger sallallaahu ‘alayhi wa sallam:
“And We have sent down to you the Book in truth, confirming the Scripture that came before it, testifying to the truth contained therein whilst exposing the falsehood that has been added therein.” [Soorah al-Maa'idah 5:48].
That Islaam is suitable for implementation in any age, any where and by any nation does not mean that it becomes submissive to nations – being altered and changed by them – as some people falsely think. But rather it means that whenever it is truly adhered to then it brings benefit and goodness to that nation, as well as reforming and correcting it – in whatever age or place.
 The religion of Islaam is the religion of truth. It is the way of life that Allaah – the Most High – guaranteed His help and victory to those who truly adhere to it, and that He would make it dominant over all other religions. Allaah – the Most High – said:
“It is He who has sent His Messenger with the guidance and the religion of truth, that it may prevail over all other religions, even if the pagans detest it.” [Soorah at-Tawbah 9:33].
And Allaah – the Most High – said:
“Allaah has promised to those amongst you who truly have eemaan (true faith and belief) and act in obedience to Allaah and His Messenger, that He will grant them ruler ship upon the earth, just as He granted it to those before them, and that He will establish their Religion for them and grant them the authority to practice their Religion which He chose and ordered them with. And He will certainly change their situation to one of security, after their fear. Providing that they worship and obey Me, not associating anything else in worship with Me. Then, whoever rejects this favour by disobedience to their Lord – then they are the rebellious transgressors.” [Soorah an-Noor 24:55].
 The religion of Islaam is a complete religion comprising both ‘aqeedah (beliefs) and sharee’ah (laws).
In short, Islaam orders alls that is good, from manners and morals, and prohibits all that is evil from it. Likewise, it orders all actions which are righteous and good and prohibits all actions that are evil and harmful. Allaah – the Most High – said:
“Indeed Allaah enjoins upon you justice, kindness and the giving of good to relatives and near ones. And He prohibits you from all shameful and evil deeds, oppression and transgression. Thus He admonishes you, that you may take heed and be reminded.” [Soorah an-Nahl 16:90].