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 Refutation against 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 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. And 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]