In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Shaikh Abdullaah Al-Bukhaari [may Alaah preserve him] said:
And from that 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] through clarification and explanation. The saying of the servant [i.e. the Muslim] لا إله is a refutation against falsehood and its people- those who deify others besides Allaah. And the statement إلا الله is an affirmation of truth and a clarification of it. It [means] that Allaah alone has the right to be worshipped and He has no partner. Likewise, the statement محمد رسول الله is an affirmation of the Messengership of the Prophet [sallal-laahu-alayhi-wasallam] and 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. Abridged and slightly paraphrased]