The kalimah is the foundation of Islam and has been the first and foremost teaching of all the messengers of Allah Ta‘ala.

Hazrat Abu Hurairah (radhiyallahu ‘anhu) reports that Nabi (sallallahu ‘alaihi wasallam) said:

عن أبي هريرة رضي الله عنه : عن النبي صلى الله عليه وسلم قال : الإيمان بضع وستون شعبة والحياء شعبة من الإيمان (صحيح بخاري #9) و في رواية مسلم من الزيادة : أعلاها لا إله الا الله وادناها إماطة الاذي عن الطريق والحياء شعبة من الإيمان (صحيح مسلم #35) 

“Imaan has more than sixty branches and Hayaa’ is a great branch of Imaan.” In one narration, Nabi (sallallahu ‘alaihi wasallam) said, “The greatest and highest of the branches of Imaan is the kalimah لا إله إلا الله and the lowest of its branches is to remove harm from the pathway, and Hayaa’ is a great branch of Imaan.”

لَا اِلٰهَ اِلَّا اللّٰهُ مُحَمَّدٌ رَّسُوْلُ اللّٰهِ 

Translation: There is none worthy of worship besides Allah and Muhammad (Sallallahu ‘Alayhi Wasallam - Peace be upon him) is the messenger of Allah.

The kalimah is the door to entering the fold of Islam. Proclaiming and believing in the kalimah immediately causes one to become a muslim. A person who does not proclaim and believe in the kalimah is not a muslim - even if he has acquired a lot of information regarding Islam. If a person, after living his entire life as a disbeliever, proclaims and believes in the kalimah, he will instantly become a muslim and all his past sins and acts of kufr (disbelief) will be forgiven. Hence belief in the kalimah is the foundation on which Imaan (faith) rests.

The kalimah consists of two parts. The first part of the kalimah contains the teaching of the oneness of Allah Ta‘ala and the second part contains the teaching of the risaalah (prophethood) of Rasulullah (sallallahu ‘alayhi wasallam).