Basics- Revisiting and Revising The Most Important Fundamentals: [The Shahaadah is a Clarification of Truth and Rebuttal Against Falsehood]
In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
Shaikh Abdullaah Al-Bukhaari [may Alaah preserve him] said:
And from which the people of knowledge have affirmed is that this religion is established on two affairs: Clarification of the truth and refuting falsehood and its people. The affirmation of this (affair) becomes manifest to the one who contemplates on the statement of Tawheed [لا إله إلا الله] with clarification and by way of explanation. The saying of the slave (i.e. the Muslim) [ لا إله – there is no deity worthy of worship] is a refutation against falsehood and its people-those who deify others besides Allaah. And the statement [ إلا الله – except Allaah (i.e. Allaah alone has the right to be worshipped)] is an affirmation of truth and a clarification of it. And it (means) that Allaah alone has the right to be worshipped and He has no partner. Likewise, the statement [محمد رسول الله – Muhammad is the Messenger of Allaah] is an affirmation of the Messengership of the Prophet [peace and blessings of Allaah be upon him] – a refutation against the one who gives him a status above that and thus deifies him or lowers the rank of his Prophethood and Messengership.
[Source:المقالات الشرعيّة page 14-15]