Imam Ja'far bin Muhammad al-Sadiq1 (a) reportedly quotes his forefathers from the Messenger of Allah (a) as saying:
“Whoever recites sincerely that there is no god but Allah enters paradise, and his sincerity should forbid him what Allah has forbidden.”2
It is well established that everything has a foundation and the foundation stone of Islam is confirming the Oneness of Almighty Allah. Whoever truthfully says, “There is no god but Allah”, indeed fixes the foundation of Islam in his heart as a builder digs the foundation for a building.
Whoever believes in the Oneness of Allah, and that Allah alone is the Creator and Provider in whose hand is death and life that He is Able over everything and He alone should be worshipped ...is a truthful believer, who is sincerely committed to Islam.
Whoever believes that there is no god other than Allah Who is All-Powerful over all things and Who punishes man for his misdeeds and sayings...does not disobey Him but avoids what is forbidden (haram) because he believes that Allah is All-Knowing of what we do and will judge us on the Resurrection Day for our actions.
This truthful belief restrains man from doing forbidden acts; he neither wrongs nor deceives anyone, nor does he steal or drink alcohol; he is neither undutiful towards his parents nor does harm others.
The Messenger of Allah (s) teaches us that Islam is a confession from the heart spoken by the tongue, that a true Muslim is one who applies Islamic methods in organising his personal and social life as well as in his public life. So, a true Muslim has truthful belief that deserves paradise and Allah's pleasure.
But whoever utters “There is no god but Allah” only with his tongue while his heart does not confess it and does not refrain from what Allah has forbidden, is not a true faithful. If he were truthful, he would not commit forbidden acts and would not disobey Allah; hence ultimately his end will be in hell.
1. Islam is a belief in the heart and doing righteous deeds in this life.
2. A true belief in Allah is belief that causes man to abstain from what He has forbidden.
3. A true belief is the base to reform society and spread the goodness and virtues of life.
4. The consequence for those who believe in the Oneness of Allah and obey Him is paradise and blissful eternal life.
Say: There is no god but Allah, and you will succeed.